In the Steam Betas tab for this game is a new 1.7 beta to fix an issue with AVX. Please try that beta, does it help you?
Does this Mac perhaps only have an Intel video system? if so that is the cause of your problem as that is not supported hardware for this game under MacOS, and since 1.6 it does prevent the game from even starting.
If that is the situation all you can do is revert to 1.5..1 , or run 1.6.x under Windows in Apple Bootcamp if you have that installed on this machine, or only play on any other computer you might have access to with supported hardware.
Sorry for the bad news!