Mac's are supposed to be this "no-brainer" way of doing things (not to say that you have no brains, eh?! 8) )
But the sound icon can be accessed from the top left hand side of the Apple menu (the Blue Apple icon) where you will see "System Preferences". Clicking on that would let you access "Sound" (in the "Hardware" category)
Check the "Input" tab to see if your "Internal Mic" is selected and the volume is set to pick up something, then clap your hands or say something to see if the indicator picks the sound up.
On the GB side, make sure you Add a "Real Instrument" track (as opposed to the "Software instrument" track.
Click on the Record icon in that track and you should be able to see the "meter" move as you say something.
Click on the Record button in the Transport window and record awaa-aay!!
QF