Please help me find the problem...

local tButton = {
	isClicked = function( self, x, y )
		return true and self.x <= x and self.maxx > x and self.y <= y and self.y + 3 > y or false
	end,
	render = function( self, bColor, tColor )
		term.setBackgroundColor( bColor )
		term.setTextColor( tColor )
		for i = 0, 2 do
			term.setCursorPos( self.x, self.y + i )
			term.write( string.rep( " ", self.maxx - self.x ) )
		end
		term.setCursorPos( math.ceil( (self.maxx + self.x - #self.str)/ 2 ), self.y + 1 )
		term.write( self.str )
	end,
}

local function newButton( x, y, maxx, str )
	local button = { x = x, y = y, maxx = maxx, str = str }
	setmetatable( button, { __index = tButton } )
	return button
end

local function getClicked( tButtons, x, y )
	for _, button in ipairs( tButtons ) do
		if button:isClicked( x, y ) then
			return button.str
		end
	end
end

local tPages = {
	render = function( self )
		term.setBackgroundColor( self.backgroundColor )
		term.clear()
		for k, v in pairs( self[ self.current ] ) do
			v:render( self.bColor, self.bTextColor )
		end
	end,
	
	handleEvents = function( self, ... )
		local event = { ... }
		if event[ 1 ] == "mouse_click" then
			local clicked = getClicked( self[ self.current ], event[ 3 ], event[ 4 ] )
			if clicked then
				if clicked == ">" then
					self.current =  self.current + 1
					term.setBackgroundColor( self.backgroundColor )
					term.clear()
					self:render()
				elseif clicked == "<" then
					self.current = self.current - 1
					term.setBackgroundColor( self.backgroundColor )
					term.clear()
					self:render()
				else
					return "button_click", clicked
				end
			else
				return ...
			end
		end
		return ...
	end,
}

function makeButtonPages( backgroundColor, bColor, bTextColor, ... )
	local maxx, maxy = term.getSize()
	local tStrings = { ... }
	local maxStringLen = 0
	for k, v in pairs( tStrings ) do
		if #v > maxStringLen then
			maxStringLen = #v
		end
	end
	if #tStrings < 1 then
		error( "Not enough arguments", 2 )
	end
	local tButtonPages = {{}}
	local x = math.ceil( maxx / 2 - maxStringLen / 2 ) - 1
	local nMax = x + maxStringLen + 2
	local y = 3
	for k, v in pairs( tStrings ) do
		if y + 3 >= maxy then
			tButtonPages[ #tButtonPages + 1 ] = {}
		end
		tButtonPages[ #tButtonPages ][ #tButtonPages[ #tButtonPages ] + 1 ] = newButton( x, y, nMax, v )
		y = y + 4
	end
	for k, v in ipairs( tButtonPages ) do
		if k ~= 1 then
			v[ #v + 1 ] = newButton( 1, 2, math.ceil( maxy / 2 + 1 ), "<" )
		end
		if k ~= #tButtonPages then
			v[ #v + 1 ] = newButton( maxx - 2, maxx, math.ceil( maxy / 2 ), ">" )
		end
	end
	tButtonPages.backgroundColor = backgroundColor
	tButtonPages.bColor = bColor
	tButtonPages.bTextColor = bTextColor
	tButtonPages.current = 1
	return setmetatable( tButtonPages, { __index = tPages } )
end

local menu = makeButtonPages( colors.lightBlue, colors.blue, colors.green, "Click Me!", "Test", "Hello", "This one is really long", "Page_2" )

menu:render()

while true do
	local event = { menu:handleEvents( os.pullEvent() ) }
	if event[ 1 ] == "button_click" then
		term.clear()
		term.setCursorPos( 1, 1 )
		print( event[ 2 ] )
		error()
	end
end

Now that you've seen my ugly wall of code I'm testing, here's the frustrating error.

[string "term"]:7: bad argument #1 to '?' (number expected, got nil)

AFAIK, this doesn't give me a reference I should be looking at, other than this is me passing a nil to term.setCursorPos. I wish I knew which line, since I could trace from there, but there's not much I can do. (Btw, is there any particular reason it doesn't give me the line number? If there isn't, could I suggest this be changed?)

Ok, figured that out. Now it's just not drawing anything.

Edit: Updated code, but now pages aren't working. More details in last post.

Might be easier to use some debug prints to a different term object (the terminal, an attached monitor, wherever) to narrow down which line is causing the error.

Update: got it to stop erroring, but it doesn't draw anything.

Okay, I think you would want it like so?

Spoiler

local tButton = {
	isClicked = function( self, x, y )
		return true and self.x <= x and self.maxx > x and self.y <= y and self.y + 3 > y or false
	end,
	render = function( self, bColor, tColor )
		term.setBackgroundColor( bColor )
		term.setTextColor( tColor )
			for i = 0, 2 do
				term.setCursorPos( self.x, self.y + i )
				term.write( string.rep( " ", self.maxx - self.x ) )
			end
		term.setCursorPos( math.ceil( (self.maxx - self.x) / 2 - #self.str / 2 ) + self.x, self.y + 1 ) --# Forgot to add self.x
		term.write( self.str )
	end,
}

local function newButton( x, y, maxx, str )
	local button = { x = x, y = y, maxx = maxx, str = str }
	setmetatable( button, { __index = tButton } )
	return button
end

local function getClicked( tButtons, x, y )
	for _, button in ipairs( tButtons ) do
		if button:isClicked( x, y ) then
			return button.str
		end
	end
end

local tPages = {
	render = function( self )
		for k, v in pairs( self[ self.current ] ) do
			v:render( self.bColor, self.bTextColor )
		end
	end,

	handleEvents = function( self, ... )
	local event = { ... }
		if event[ 1 ] == "mouse_click" then
			local clicked = getClicked( self[ self.current ], event[ 2 ], event[ 3 ] )
			if clicked then
				if clicked == ">" then
					self.current =  self.current + 1
					term.setBackgroundColor( self.backgroundColor )
					term.clear()
					self:render()
				elseif clicked == "<" then
					self.current = self.current - 1
					term.setBackgroundColor( self.backgroundColor )
					term.clear()
					self:render()
				else
					return "button_click", clicked
				end
			else
				return ...
			end
		end
		return ...
	end,
}

function makeButtonPages( backgroundColor, bColor, bTextColor, ... )
	local maxx, maxy = term.getSize()
	local tStrings = { ... }
	local maxStringLen = 0
	for k, v in pairs( tStrings ) do
		if #v > maxStringLen then
			maxStringLen = #v
		end
	end
	if #tStrings < 1 then
		error( "Not enough arguments", 2 )
	end
	local tButtonPages = {{}}
	local x = math.ceil( maxx / 2 - maxStringLen / 2 ) - 1
	local nMax = x + maxStringLen + 2
	local y = 2
	for k, v in pairs( tStrings ) do
		if #tButtonPages[ #tButtonPages ] * 4 > maxy then
			tButtonPages[ #tButtonPages + 1 ] = {}
		end
		tButtonPages[ #tButtonPages ][ #tButtonPages[ #tButtonPages ] + 1 ] = newButton( x, y, nMax, v ) --# Here
		y = y + 3
	end
	for k, v in pairs( tButtonPages ) do
		if k > 1 then
			v[ #v + 1 ] = newButton( 1, 1, math.ceil( maxy / 2 + 1 ), "<" )
		end
		if k < #tButtonPages then
			v[ #v + 1 ] = newButton( maxx, maxx, math.ceil( maxy / 2 + 1 ), ">" )
		end
	end
	tButtonPages.backgroundColor = backgroundColor
	tButtonPages.bColor = bColor
	tButtonPages.bTextColor = bTextColor
	tButtonPages.current = 1
	return setmetatable( tButtonPages, { __index = tPages } )
end

local menu = makeButtonPages( colors.black, colors.blue, colors.green, "Click Me!", "Test", "Hello" )
term.clear()
menu:render()

while true do
	local event = { menu:handleEvents( os.pullEvent() ) }
	if event[ 1 ] == "button_click" then
		term.clear()
		term.setCursorPos( 1, 1 )
		print( event[ 2 ] )
		error()
	end
end

In line 12 of the drawing function you actually forgot to add the first x-coordinate to the cursor position - leading to the text being at the left of the screen.
Other that that your error was using tButtonPages[ #tButtonPages ] instead of tButtonPages[ #tButtonPages ][ #tButtonPages[ #tButtonPages ] + 1 ] in line 84.
This was leading to buttons being created as pages - you forgot to put the buttons inside the tables.
You might also increase y by four on every addition to leave spaces between the buttons.

-snip-

…Working on it. I figured out the drawing problem.

Update: I got it working, except my right/left buttons aren't working for some reason. I've used some error statements to see what has happened, and it is correctly inserting a new button into the table. I haven't been able to test the left button, but the right button is defiantly broken.

Not sure if this is your issue, but this is your button constructor:

local function newButton( x, y, maxx, str )  -- No mention of maxy.

And these are the calls you're using to build the arrow buttons:

                        v[ #v + 1 ] = newButton( 1, 2, math.ceil( maxy / 2 + 1 ), "<" )  -- ... maxy?
                        ...
                        v[ #v + 1 ] = newButton( maxx - 2, maxx, math.ceil( maxy / 2 ), ">" )

Speaking of which, instead of having an extra loop for those calls, perhaps move them up into this block:

        for k, v in pairs( tStrings ) do
                if y + 3 >= maxy then
                        tButtonPages[ #tButtonPages ][ #tButtonPages[ #tButtonPages ] + 1 ] = newButton( maxx - 2, maxx, math.ceil( maxy / 2 ), ">" )
                        tButtonPages[ #tButtonPages + 1 ] = {}
                        tButtonPages[ #tButtonPages ][ 1 ] = newButton( 1, 2, math.ceil( maxy / 2 + 1 ), "<" )
                end
                tButtonPages[ #tButtonPages ][ #tButtonPages[ #tButtonPages ] + 1 ] = newButton( x, y, nMax, v )
                y = y + 4
        end

My creating function takes x, maxx, y, and text. I am attempting to position it halfway down the screen, so I give it a y of half maxy.

I think I'll take your suggestion about the loop though.

Maybe you should take another look at the order in which it takes those parameters, and the order in which you're supplying them.

*facepalm*

I am so stupid. How did I not notice this?

…

Fixing stuff in the code now.