In the game's Main menu, go to contents manager, and in the mods category select whatever mod you want to activate. You may get a message warning you that the mod isn't supported by either of the game's companies. And just to give you a heads-up, some mods, like this one, will disable game achievements. You can find a mod that will re-enable steam achievements while the other mods are activated, although I haven't tried out that particular mod myself.