Skins?
As in... let's say you have a new skin for a Paladin... and you want your new skin to show on him instead of his normal one.
Open WorldEdit, press F12 to open the import manager.
File -> Import -> (name of your skin) -> hit ok.
Then, change the name of the skin to the name of the normal Paladin skin (this should overwrite the original skin).
If, how ever you have a model and a skin that you wanna import then hit F12 ->Import File ->(Import the skin and the model that you want) -> Double Click the file you just imported, click custom path remove the "war3..../" from the file so you have B(modelname).mdx and (modeltexture).blp
Then simply go to the unit you want to have the skin, click 'model' go to Custom and it should be there.
(lol... this may be all wrong ><; Im new to it myself)