Log into the account management section on the wow website, with the correct wow arrount of course.
You should have a big, green, "Upgrade Now" image. Click it.
on the next page, read the text and find "If you have already purchased a retail copy of the Burning Crusade, you do not need to purchase it again and may go to the Burning Crusade Authentication Key Upgrade page. " - Click that link and you'll be on a page where you can put in your BC license key.
The page is https://www.worldofwarcraft.com/acco...e-upgrade.html

Hope this helps
-Zub