I've heard so many people ask this question. Alright garaa, i believe what you want to ask is how do you import a skin into a game.
Well you see, models and skins are different.
A model is a whole new unit.
A skin is just a texture that replaces an old units texture.
For example, a demon footman skin. If you put that in your game it would replace the OLD normal footman skin with the demon one.
So lets see how we can put this into a game:
For this i am going to use my (Lovely) Jungle marine skin.
http://hiveworkshop.com/resources_new/skins/963/
Notice how its the same shape as the marine, just a different skin.
See the path i have provided?
Units\Critters\Marine\SpaceMarine.blp
Thats the path for the normal marine which is included in game. Giving the skin this path after importing it will replace the old skin with this one.
Now, open world editor.
go to the IMPORT MANAGER.
Then right click and go to IMPORT FILE.
Now import the file.
Right click on it, go to Modify file properties.
Change it to the path we have.
Which is Units\Critters\Marine\SpaceMarine.blp.
Now save your map, close it, and re-open it.
PRESTO! THERES YOUR SKIN!