You probably have some mods made for regular AoM, which change files that The Titans is supposed to use. Try deleting all the .xmb and .xml files in your Age of Mythology\data directory. If that still doesn't fix it, uninstall both games, delete your Age of Mythology folder to delete any mod files that might've been left there, and reinstall the games.
When your game is working properly, you should see a big Titan Gate icon appear under your god power icons once you reach Titan Age.