Sorry to say, but, AFAIK, there is no way to play multiplayer civ 3 on a Mac. In order to play multiplayer civ 3 to begin with, you need the 'Play the World' expansion pack. Unfortunately, it hasn't come out for Mac yet, although I believe they have plans for a port. From what I've heard, though, the Multiplayer is extremely slow/buggy, so it might be good that they haven't released it for Mac yet. Might give them a chance to fix all the bugs before the release.