Mods are a bit hit and miss right now in Civ3 Complete.
Since I posted the above, it has become apparent in another thread that if you try to use the installed (PtW) versions of the terrain graphics mods you get some rather violent geothermals as a "bonus" - the volcanoes appear to go hyperactive

It is reported that you *can* download the C3C versions of these terrain mods and they *do* work OK, but you'll need to find out how to install them.
Installing mods is another minefield.
The intention is that you should be able to place mods and scenarios in the Civ3 Complete and Play the World Scenarios folders that are automatically created in your ~/Documents/ folder. Thids is a worthy objective, as it aligns well with the Mac OS ethos that user data goesn in user home folders and Applciations should be read-only.
However, it appears to have backfired, and few mods and scenarios actually work when placed there ... or even when placed in the Scenario folders in the Application. It appears that more complex file path translations are needed to cope with the changes Aspyr made to the folder structures.
The changes required are in a patch that Aspyr are planning to release "Real Soon Now" (TM). So it may be worth while to hold off trying to customise your Civ3 Complete installation to within an inch of its life until after that patch is released.
If you really can't wait then I'm sure there will be willing helpers here to try to get you going, but be prepared for a bumpy ride. My solution so far has been to use Brute Force. I do it the same as I did in the old MacSoft version - replace the basic /Applications/Civ III Complete/Civlization 3 Game Data/Art and /Text files with the ones from the scenario I am trying to use. But that means you have to be prepared to back up files you are changing, and swap them back afterwards, or reinstall the game to get back to the standard version.