Well, they've got in touch and here's a solution....

You probably entered an IP address in the IP address field in the pre-game
settings menu that appears before the game launches to the full screen. If
you no longer see that window, you can invoke it by holding down the Command
key while double clicking the game icon. Remove the IP address and all
should be good.

MacSoft have seen a couple of instances where the game still
behaves in the same way. In these cases, drag the Halo Preferences file in
your home /Library/Preferences folder to  the trash. You will need to
re-enter your CD key when you start the game again, but this will take care
of the problem.