With the recent release of patch 4.0.1 many add-on authors may have noticed that the AddOn Kit will no longer automatically extract the lua/xml and art files from the player’s installed version of WoW. These files are now available on the World of Warcraft site in our Technical Support pages
here and can be downloaded directly.
Please note, this is for AddOn Authors, not end-users. It is important to be aware that customizing the interface can be a very technical endeavor, and you should not attempt it unless you have a good working knowledge of XML and Lua.