Adam,
The FAST_SFunc.mexw32 only works on 32-bit Windows platforms (this is what the “w32” in the mex name stands for). This mex file will not work on a Mac OS X.
I don’t run a Mac myself, so, I can’t help you out much. But the post found here: [FAST S-Func MEXing process) describes how to compile the FAST_SFunc.mexw32 file. Hopefully with the information found in that post, as well as with the MATLAB documentation, you can learn what you need to in order to compile it for a Mac OS X. If you succeed, please post on what you find out so that you can help other users with similar problems.
Best regards,