Problems with gmod while using mac

I was playing DarkRP and appeared a message saying to press f1 for help, but when i press f1 appars brightness option from mac i dont know more what to do please help (sorry for the bad english i m brazilian)

on your mac go to:
system preferences >keyboard>keyboard> and click the button that says “use all f1,f2etc. keys as standard function keys…”

you will now need to hold the fn button and press the F keys to change the brightness etc.