Starting with the next Mists of Pandaria beta build, we plan to enable the use of UI addons. So whether you’re a savvy addon author or the most discriminating player, there’s no time like the present to start planning out your perfect UI.
Addon authors should also make sure to check out prominent addon sites such as WoWInterface
for a chance to apply for a Mist of Pandaria beta key. These sites have recently been given a lot of beta keys to distribute specifically to active addon authors. We likewise strongly encourage authors who have access to the beta to start testing out and fine-tuning their addon updates in the beta as soon as possible.
If you use an addon and start seeing errors, try downloading the most recent version of it. If you are still having errors, you may need to turn off the addon until it has been updated.