If you purchase one of the two in-app skins you must do the following to apply the skin:
1) Open the SmartStart app
2) Click on the "cars" tab
3) Click on the blue arrow next to your car
4) Click on "home page skin"
5) Select one of the available skins from the on-screen list
Issues involving already paid for and downloaded skins can typically be resolved in iTunes. Look for your app purchases, and a link that says "Resolve a problem".