It's a Macbook? Does it have an Nvidia or ATI card built-in? Because if it doesn't, I'm surprised it was even able to play Portal. Every person I've ever talked to on the Steam forums that tries to play portal on a Macbook (or a PC with integrated video) has trouble. Most of the time it's because of crappy integrated Intel graphics chips that support DX 9.0c, but don't support shader model 2.0 (I think that's what it is) in hardware. Software rendering simply won't do it for Portal.

HL2 and HL2:Ep1 technically have slightly lower requirements than HL2:Ep2, even though they're all in the same box. So it wouldn't be a surprise if you could run those and not Portal.