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
Nvidia geforce GT 8800 or ATI radeon 4850 or better
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...
If Assassins Creed 1 runs anything like 2, I wouldn't recommend anyone buy it for the 2010 13" MBP.
It runs ok (its playable), not anything like the ps3 or xbox 360 which pretty much ruins the experience.
But if you lower the graphic performance on the game, maybe your 13" will play the game just fine.