As Vaeflare hints in this blue post, do bear in mind that addons that work beautifully on live servers will likely not work in beta. Mists reworks a great number of game mechanics, so the addon creators are working almost as hard as Blizzard themselves at getting their programs up to scratch.
If you're trying out an addon and it's not working, first make sure that you have downloaded the Mists beta version, not the live version. Second, make sure that you have downloaded the latest beta version from the developer's site, or check Wowinterface or Curse for the latest versions.
Remember that these addons are in beta themselves and that any bugs you find should be reported to the addon developers via the appropriate channels, not to Blizzard.
It's open warfare between Alliance and Horde in Mists of Pandaria, World of Warcraft's next expansion. Jump into five new levels with new talents and class mechanics, try the new monk class, and create a pandaren character to ally with either Horde or Alliance. Look for expansion basics in our Mists FAQ, or dig into our spring press event coverage for more details!