font_type = FONTS.SMALL

function mytestmess(str, pos)
    draw_centered_text(str, pos, font_type)
end

add_hook ("new_game", "test", mytestmess("Kool-Aid man says: Oooh Yeah!", 100))
add_hook ("joint_select", "test", mytestmess("You like selecting joints?", 100))
add_hook ("enter_frame", "test", mytestmess("Next frame, please!", 100))

Shouldn't the above code work? I don't get any errors, but no text appears. Also, I can't find the stderr.txt anywhere...

add_hook() takes three arguments. You can find it in startup.lua in your scripts directory.

The third argument is a function reference, and you're passing it nil, because mytestmess() doesn't return anything.

If you want, you can use a function literal like:
add_hook ("new_game", "test", function() mytestmess("Kool-Aid man says: Oooh Yeah!", 100) end )

Hum, thanks for the help, but still nothing happens. Any thoughts about Stderr.txt?

The 'stderr.txt' should appear on the Toribash main folder.
Use 'echo' for debugging, as in

function mytestmess(str, pos)
    draw_centered_text(str, pos, font_type)
    echo(str)
    echo(pos)
    echo(font_type)
end

I don't know about the draw_centered_text function.
You might have to set the text color beforehand, i.e.

set_color(0,0,0,1)

before calling your function.

Good Luck

Stderr.txt still doesn't show up: Also, the echo's return the correct information (or so I think), it just looks like draw_centered_text isn't working...

you can only draw stuff in a draw2d hook.

If so, then the solution could be the run_cmd and "cp" commands, ins't it?

function display(str)
    run_cmd("cp "..str)
end

Warning: i can't check if it works at the present time, so IDK if the sintax is ok.

center print != draw_centered_text
Works:

Message = ""
Pos = 0
Colour = {r = 0, g = 0, b = 0, a = 1}
font_type = FONTS.SMALL

function mytestmess(str, pos,r,g,b)
    Message = str
    Pos = pos
    Colour.r = r
    Colour.g = g
    Colour.b = b
end

function draw2d()
    if(Message ~= "") then
        set_color(Colour.r,Colour.g,Colour.b,Colour.a)
        draw_centered_text(Message, Pos, font_type)
    end
end

add_hook("draw2d","moo",draw2d)
add_hook ("new_game", "test", function() mytestmess("Kool-Aid man says: Oooh Yeah!", 100,1,0,0)end)
add_hook ("joint_select", "test", function() mytestmess("You like selecting joints?", 100,0,1,0)end)
add_hook ("enter_frame", "test", function() mytestmess("Next frame, please!", 100,0,0,1)end)