If you play in Window mode you can pause the game using command-H, or simply click out of the window it's running in if it isn't filling the whole screen.
To get to Window mode you need to hold Command down while you launch the game. You get a dialog box where you can select screen resolution, wondow or full-screen mode, and some performance options. You can also check a box so that this screen comes up each time you start Complete.
Aspyr have a patch forthcoming that will add a pause facility for full-screen mode.
PS In Window mode you can also use Command-Tab to bring up the application switching toolbar and swap out of Civ3 that way.
