Make sure you've downloaded the pack right, and remember to replace the old .CFG file with the new one from the MM3 folder.
Remove all your markings from the textures folder and paste in the new ones, then open your game and you should be done.
Here is a video that you can follow and install it correctly. (:
http://www.youtube.com/watch?v=gPtD6SE3Bng (http://www.youtube.com/watch?v=gPtD6SE3Bng)
Video isn't mine
Ah, try this one.
Don't mind the 'it doesn't work' comments at the start.
http://www.youtube.com/watch?v=3VqZUe5HD-U (http://www.youtube.com/watch?v=3VqZUe5HD-U)
Vid isn't mine either