For all Mac os x Lion users and specially the one who update to 10.7.2 here's the solution to run Assassin's creed brotherhood.
Go to Applications/Ubisoft/Assassin's creed brotherhood folder.
Inside the folder u can see the application right click on it and choose "show contents". go to Contents / MacOS and DoubleClick "acbmp_sf" a terminal window appear and now u will be cable to connect on ur ubisoft account, u have 30 seconds after you connect to choose offline mode or solo mode before the application exit. when u choose solo mode the application will exit and then restart normally and now u can play assassin's creed brotherhood.
Have a nice day guys