This Battle.net CD Key requires an European IP-Address. This can be done through VPN if you’re not in Europe. Activate the game by clicking on “Account Management” and then on “Add a new game”. Enter the code there and have Wow Mists of Pandaria ready for download. Read the store’s description if you need help.
A Wow Mists of Pandaria key for the Battle.net-Client does not expire and after activation Wow Mists of Pandaria can be downloaded and played forever.