Actually if you go through their options and disable the World of Warcraft official schema on their own files, then you will have full access ot it. As of the 1.9 patch, they deviated from their own schema, and the original settings for the program cough and don't work.
If you check out the guy's home page on the topic, he lists exactly who-what-where-when-why-how to do so.
If you are only worried about getting a working LUA editor, I'd seriously recommend getting ahold of textpad.
http://www.textpad.com is a good start (check under downloads->addons->syntax libraries to get a LUA syntax highlighting addon)
But yeah... TFM can be found at
http://www.wowwiki.com/Interface_Customization and the key thing to remember now that you can find it is to take a few and RTFM. They list many other alternatives for IDEs there as well.