In the beta test of the upcoming Mists of Pandaria expansion pack AddOns were completely disabled to allow Blizzard to discern between client errors (bugs in their own code) and third-party errors (bugs in addon code). It has been a few months since the beta was opened up and in the next beta update, addons will be re-enabled to allow addon authors, including myself, to make sure their projects will work when the expansion pack goes gold.
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 and Curse 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.
I personally have avoided playing the beta, as I have done with all previous beta tests. I have a monthly download limit enforced by my ISP and the amount is simply far too low to allow the beta and every subsequent update to be downloaded.
I will have to rely on my customers to report any errors they come across, if any. Most of my addons should work fine but if you do find an error, feel free to contact me on Twitter or file a bug report on GitHub, WoWinterface or Curse. I read every message and investigate every bug report I receive.