Alright, ill go through it step by step.
Download this skin file, and put it some where you remember
http://hiveworkshop.com/resources_new/skins/18/
(Ill use this as an example)
1. Open World editor
2. Open the import manager (The button with the A on it on the top tab)
3. Right click or go to File and select IMPORT FILE
4. Choose the skin that you downloaded
5. The skin file should appear in the import manager, right click on it and click Modify File Property's.
6. Check the box that says Custom Path.
7. Put in the required path for the skin.
E.G
The skin is based on the demon hunter, the path should be
Textures\HeroDemonHunter.blp
Here is a sample map and picture's of what it should look like in game.