Download the World of Warcraft Interface AddOn Kit
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. http://us.blizzard.com/support/article.xml?locale=en_US&articleId=21466
*It is important to note 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. There is no official support
for modifying the WoW interface.