While there might be other Mac users in the community which would be able to help you, it's probably best to contact GOG's support about this issue. This might also lead to patches for future players. :)
You should also tell the person you expect help from something about what setup you're using* as well as what you already tried to solve the problem, in order to speed the whole process up.
Best of luck!
* As someone outside of the Mac community, I would assume that there are still differences between your machines.