I don't think there is a mac version currently for the beta, hopefully Simon will make one soon.
The beta is TPT rewritten in C++, so there might be something different about mac compiling, or there would be a download available. Just wait for the full release (unless Simon makes a mac beta build before that).
Because Simon doesn't like compiling for mac. 'nuf said.
The beta is there because the server responds with "Hey, new beta avalible! But there is no compile for mac, so it fails because it can't find the file.