That doesn't make sense..
local x,y = guiGetScreenSize()
dxDrawText ( "Ahoj",x/2,y/2,x/2,y/2,tocolor(0,255,255,255),1.0,"bankgothic","left","top",false,false,false)
This will draw the text at the centre of the screen (it will start at the centre).
Btw. you got error in your addEventHandler... check the syntax on wiki.