Hi
You have to asign those custom-skins via the FMB (Full Mission Builer) of the game. You can use the FMB via Main-Menu--->Extras.
You have to build yourself a mission to use the skins, which is pretty doable. There are some FMB-Tutorials on Youtube.
If you need further help, I suggest asking at the ATAG-Forums:
www.theairtacticalassaultgroup.com. This forum is not so much alive anymore...
Cheers!