You will need to put the applicable files into the Assets/Civilizations/Civ4leaderheadinfos
and the Assets/Civilizations/Civ4Cilizationsinfos files, that's assuming you've got not unique units and jazz, if so you'll need to fool around with other stuff as well. Just make a new mod folder, make a folder called assets, then inside that Civilizations, and then those files i mentioned above, and copy-paste the stuff you downloaded into the files.
don't touch your original files!