This is an extremely common Mac error. There are threads all over the Official forums and on MTS about it. Try re-naming your The Sims 3 folder to something else and starting a completely fresh game. Otherwise, Google might help. That's all I know on the subject because I don't have a Mac and have never experienced the problem.
If I take AM out of my Mods folder, I can load CAS just fine. I went through the config tool to double check the options I chose, and everything's the same as on my Macbook. I thought maybe it was because I was in a custom world from MTS, so I tested it on an EA made world and I got the same error. I'm not surprised that it's a common error on a Mac, but I'm really confused on why it's happening with only AM installed. I don't have any other mods installed at the moment. I'll try and call EA's customer support and see what they have to say, but it seems like their service is a bit dodgy. Just hoping I get the error figured out and can use AM.
However, I will try a fresh game and see about moving AM into it.