you don't
to my knowledge there are no itunes skins... i don't think finder skins would affect itunes either. there is probably a de metalizer - but that isnt a skin it's a script that changes a plist value to make the app aqua. you can do it with interface builder i tink, at least for older metal apps.
Those seem to owork in iTunes 3. But try if they work .. if they wouldn't work, I'm sure the iTunes 4 skins will be out soon. Try, and tell if they work on 4 as well.
There is a tutorial on resexcellence for installing all their skins. I'm guessing you'd need to use ResEdit for iTunes (whereas you can use a program like Duality for the Finder).