I believe that on Macbooks, the graphics card is not good enough for the game. The game is demanding on the system visually, and I found a site (http://www.game-debate.com/games/index.php?g_id=47&game=Saints%20Row%202) which allows you to see whether the game will work based on your specs.
The results say that with a better graphics card it will work. I used my Mid 2010 13" Macbook Pro (2.4 GHz version) 's specs.
(See the attachment link for the actual results)