Civilization IV: How to play multiplayer with Mac and Windows?

Solution 1:

When you install through Steam it appears that the default Mac OSX version of Civ IV and the Windows version of Civ IV are different.

This is the fastest way I have found of setting up a game:

1. On the Windows machine, find 'Civ IV' in Steam and right-click
2. Click on the Betas tab
3. Under 'Select the beta you would like to opt in to', click on the drop-down menu and select Original Release – UNSUPPORTED
4. Let Civ update, and launch when finished
5. Now when you launch the game, both players should have the Multiplayer -> Direct IP Connection option. Use this to have one player host a game and the other player connect.

See also the 2k announcement about installing pre-Steamworks Civ IV here.

It may be possible to also get both players running the same version by purchasing the Civilization: Beyond the Sword expansion pack for both players, but I haven't confirmed this.