I dunno if it'll work on your Mac,horatio, But you may or may not have the same keyboard as the low-life windows-users do.
Above the Arrow Keys, there are six buttons (Insert, Home, PageUp, Delete, End, PageDown) Above that should be three more.
Printscreen/Prtscn
SystemRequest/SysRq
(Then Scroll Lock and Pause/break)
Press that Print Screen thing button. Whatever is on your computer monitor at that time will be in your clipboard as an image.
This works for games, emulators (Handy for ripping. ;D) screenshots, happenings, stuffs like that. Thats how you do it.