Blizzard has announced before this weekend that they have sent around 400.000 more Mists of Pandaria beta invites and everyone that signed for the Annual Pass should have received their invite by now.
Be sure to check your battle.net account as they didn’t sent e-mails to everyone.
Login to your battle.net account, click on the PTR link, there should be “World of Warcraft®: Mists of Pandaria Beta”, WOW3 displayed there.
Click on that tab and download the client, around 12 gigabytes of data.
There isn’t much to do in the beta except starting a monk pandaren, the other characters will just start like normal from level 1.
You can also copy characters from your normal World of Warcraft account, check the “Beta Character Copy” from the link inside the Mists of Pandaria client download page.
The Beta Character Copy feature works at the moment, select the desired character you want to copy, this will get randomly assigned to different beta realms (there are around eight realms up right now).
It might take a while for the moving process to finish, you can see the status from “History/Status”, there is a link, it should say Pending if you character is still on the moving process.
You won’t have the old keybinds from WoW Cataclysm, you can either keybind everything again or copy some WoW folders (WTF folder, Account, one more double click on your account, copy all the files and folders from there inside the same folder of Mists of Pandaria).