Although there Is a 'Print Screen' key on the keyboard (not always marked as such), it's the F13 Key, the usual Screen Snapshot combination is option(the key with the apple symbol or four-looped symbol) together with the Shift key and the 3 key simultaneously.
That is, unless you have preprogrammed your Function keys at the top.
Also check that your keyboard is appropriate to the Country of origin. ie if you run a US System, select the system profiler and you should be able to see which you have, match them together by changing the keyboard layout to US and it should work as labelled.
I had a similar problem using a windows keyboard on my Mac!
I have a PC with a MAC keyboard. Since there is no "print screen" button, the PC won't do a screen grab. Is there a way around this?