The World of Warcraft Annual Pass promotion ended May 1, and we recently sent out the remaining Mists of Pandaria beta invitations. Check your games list in Battle.net Account Management
for the Mists of Pandaria beta. You can download and install it from there.
Again, thank you for your enthusiasm and feedback. To provide feedback for the Mists of Pandaria beta, we invite you to please post in the Beta Feedback forum