use the mac screen capture button (thats what I assume becuase I saw many fullscreen pictures from mac)

 

I assume you need to hold down the FN (Function) key. Pretty sure you have one... Computers often have functions in their F keys now, with a FN key to use them. I'm on a PC, but I have to hold down my FN key to use the F key functions; or the other way around (their numbers / other functions) if I had it set that way.

 

I have a mac too. It's fn f2, fn f3, etc. (fn is in the bottom left corner of the keyboard)
Hold it down then press the button.

 

Just press:

FN+F2

at the same time.