Sorry to be the voice of doom here, but unfortunately, we cannot currently transfer licenses between the Mac and the PC. As kq76 intimated above, the heart of the problem is the fact that different companies handle our licenses for the different operating systems.
Once we're past the "return period" for a product, we cannot process these, because we have to essentially tell one licensing company to give their profit away, say six months after the fact, and not due to any fundamental (e.g. "return-worthy") problem with the game.
Part of this is why we're working on a new licensing platform for all of our products that would enable this sort of sharing between OSes, but for now, I'm afraid I can't offer you any alternatives.
Steve