Go through all the choices you can pick for your avatar. What you unlocked sounds like it should be under "accessories." Normally, certain accessories have a padlock on them. When you unlock an item, the padlock icon is open, that is, unlocked. Once unlocked, you can then choose that item for your avatar.
Your unlocked item should be there with an open padlock for you to choose. Try browsing all the pages in "accessories" and see what you find. Then post your search results here if you need further assistance