What do you mean, add a path? You want the skin to have a path?
Skins do not have paths, models have them. In other words, in a model there is a part that refers to where the skin is located on your computer. So, if you would have a Footman model, it looks in the war3.mpq archive and finds a file Textures and inside it it looks for a file named Footman.blp. The World Editor has a specific order in to look for files. First it looks in the map itself, and then it looks in the MPQ archive. So, when you run your map that contains a footamn model placed somewhere, first it looks into the map for a footman.blp file in a Textures folder, and if it isn't there it looks in the MPQ archive. Thats why you are able to import skins into a map.
Cba81well i made a skin and its for the paladin that has black hair well i tried to put in path Units\Human\Heropaladinboss\Heropaladinboss
and it dose not work plz help me
You can't write another path for the paladin skin. The path has to be the same like the skin you edited, so you need the path for the normal paladin skin. If you wanna use both skins, read oz02's tutorial about how to use more then one skin for a model (tutorial-forum).