How to download aoe3 on a Mac?

So I love the aoe series and the total war stuff and decided to start getting back in to it recently when I bought a pc.

Unfortunately my brother (who lives separately to me) has a Mac (don’t have a clue why). He decided to download steam last night, then pay £30 for the game for it not to work.

Does anyone have any idea how to get the game to work from the stage he is at? I’m not great with computers but can hold my own, so if anyone has a dummy’s guide to help that’d be much appreciated!