Here are the minimum system requirements for Assassin's Creed: Brotherhood on MAC:

Supported OS: Mac OS X 10.5 or OSX 10.6 (only)
Processor: 3.06 GHz Intel Core2 Duo (Intel Core i7 recommended)
RAM: 3 GB
Video Card: 256 MB dedicated video card with Shader model 3.0 or higher (see supported list*)
HDD Space: 9 GB
Peripherals: keyboard, mouse

*Supported video cards at time of release