:state=0:hudreset,hudx=50,hudy=60,hudimagefine=(hud location),hudname=hudtext,hudhide=1,hudmake=display,state=1
:state=1,scancodekeypressed=(key number):state=2
:state=2:hudshow=hudtext,state=3
:state=3,scancodekeypressed=(key number):hudunshow=hudtext
press the key to view message, again to hide it.. if u wanna assign it to an entity add (plrdiswithin=40) to the first and third state.. if u wanna assign it to a trigger zone add (plrwithinzone), if u want message to display if he presses the button again add (,state=1) to the last line..
TIP: u might wanna read th official community guide it'll help u with these little scripts plus it'll give u a lot of experience scripting..