I have a Macbook Pro 13" and pretty much all the requirements asked on the Mac App Store are met in order to play AC2.
It says at the top of the page though,
This app is not compatible with Macbook Air and Macbook Pro 13"
Will it still work?
You should check your graphics card... The minimum AC2 requires is
I wouldn't buy it if I were you, chances are you will have the play the game with a huge lack or not at all as some users seems to complain about...
Some people share my opinion in not buying the game...
But if you lower the graphic performance on the game, maybe your 13" will play the game just fine.