Okay, first download the desired skins and models and place them somewhere safe. (Such as the Maps Directory in Warcraft III)
Then, using a tool that can unzip folders (most Windows Programs have a default one. If you don't, use Google to search for a downloadable one.) and extract the folder somewhere safe.
Now, open up World Editor and press F12 to open up the Import Manager.
Click on the "Import File" icon at the top of the manager, browse through your computer and click on the skin/models to import them.
Once this is done, open up the Object Editor and open up the unit you want to put the skin/model on.
Now, scroll down the the editor until you reach "Art - Model File" which is near the top. Double-click on it and click on the white bar next to the "Import" text, and click on the model/skin you downloaded.
If this doen't work, change the File Path name of the imported skin/model in the Import Manage by double-clicking on the skin/model file and ticking the "Use Custom Path" box and insert your own name. Then, try again.
I hope this helped you.