Here’s the latest .exe patched for addon support. Download: http://dl.dropbox.com/u/120650/MoP_beta/exe/WoW_15650_Addons.rar.
DISCLAIMER: As always be careful with files downloaded from the internet. Also, keep in mind that it’s against Blizzard’s Terms and Conditions to use 3rd party modifications to their game code and they may revoke your beta access and the usual crap bla bla bla. Use at your own risk.