Steps:
1. Download the skin that you want.
2. Extract the skin from the Zip file.
3. Import the skin into the map.
4. Set the CORRECT PATH of the skin.
Example:
If you have a skin of a grunt, you must type in the Custom Path the path for the grunt. To see the path for the grunt, go to the grunt unit, and check it's model. In the Custom Path it should be Units/Orcs/Grunt... (or something like this). Copy the path of the grunt and Paste it in the the path of the skin that you imported.
5. The skin is applied, and it should be working now. If the skin isn't showing up, review the path and see if everything is right.
-Rui