Download, unpack with .rar like into C:\Program Files\Warcraft III\Model Map, that's what I have to dump my models somewhere and then go to World Editor, Import Editor, import all in that map and delete the ReadMe, no one reads ReadMe's and delete yes the war3imported\ away from it, save the map, restart WE and go to like Object Editor, make your unit and to go Model thinger, you see then the middle tab is available (there are the imported models) and now you can use those models.
With skins it's a other story, ask someone else for that.