WINE is a free open source program which enables Windows programs to be run on *nix systems. In other words, it acts as an interface between your Windows program (MWS) and your Linux-type operating system (OSX). I didn't get it to work perfectly, but I did manage to get to the point where I could at least run decks, even if players' life totals didn't show up.
Here's Wine:
http://www.winehq.org/Good luck. If you get MWS running perfectly on it, please post and let everyone know. They're up to a newer version of WINE than when I tried it.