So far as we can tell, Steam recently updated the Steam client in a manner that seems to have goofed up the game for some Mac users. We are hoping that Steam fixes the issue soon.
Until this is fixed, please try disabling the Steam overlay. To do this, first right click Road Not Taken in your Steam library, then uncheck the Steam Overlay option in the General tab.
(Note: the picture shows the box as checked. You should uncheck it.)
If that doesn't help, please report the problem to us as described on this page and we'll try to figure out what is going on.