First you need to crate a dummy user to register the game (really hate this), than you come to the start screen. The game does not respond to the Enter key on Apple keyboards, maybe you have to buy a PC keyboard, maybe they did not even test the start screen.
When you click with the mouse on the Enter text you can go to the options, but you can never come out of the menu again because the backspace key is not recognized from the game.

This could be the best game ever - the storyline is fabulous, the characters and challenges imaginative and interesting. But dont take too much notice of the system requirements - they are ABSOLUTE minimum - My Macbook Pro at 2.4 ghz couldnt seem to synch. the voices - or cope with the constantly moving camera angles and jerky motion - which in the case of this game makes some of it virtually unplayable.

Just bought this game recently and played it on my macbook pro 13" i5 2.5gz 4gb ram and standard Intel HD Graphics 4000 512 MB.
Game worked perfectly (although not on the highest graphics but it did not look bad at all :D), very little lags which did not severely hamper the gameplay. Mac got a bit warm but nothing extreme.
I totally recommend it because it is a very fun game to play. just too bad that you can not actively travel between cities because the main part of the story takes part in rome (btw, Rome is huge)