I don't know about the 13 inch model. That machine is known for having less-capable graphics hardware. Of course, L4D2 isn't exactly a cutting-edge game, either, and it's based on an engine that was used in older games as well. (Not that I mean that as an insult, I think that all of Valve's games based on that engine are great... the graphics are quite good, the physics are great and, most importantly, the gameplay is awesome.)
Here's what its system requirements are:
OS: MacOS X 10.6.4 or higher (Snow Leopard Graphics Update required)
Processor: Dual core Intel processor, 2GHz or better
Video Card: ATI Radeon 2400 or higher / NVIDIA 8600M or higher / Intel HD Graphics 3000
Not supported: OS X 10.5.x, ATI X1600 or X1900 graphics, NVIDIA GeForce 7 graphics or Intel graphics less than Intel HD 3000
That does seem to include your machine, so it may be worth a try.