Left4Dead freezes and lags on late 2008 macbook pro

Hey, I have a late 2008 unibody macbook pro, and the specs seem to live up to the system requirements for Left 4 Dead (the first one), yet, once I get to the actual gameplay, the game freezes for about a minute, unfreezes for a few seconds, and freezes again. I keep reading about updating graphic card drivers, but I can't find a way to do that. Am I misreading the specs of my computer in comparison to the system requirements? I really didn't think the game would run this poorly on my computer. There is also no sound playing in the game. I only hear sound in the cutscenes and on the menu.