Here's a good place to start:

http://www.hazardx.com/dbts/files/TREYscripting.htm

This link leads to some documents explaining various things, like how scripts work, what the various files do by extension, and some other things.

Also, feel free to drop by on HazardX's forum. :) That's where most of the modding is taking place and has the Xtended version, Cusspack, and some modding tools like ATDpack, which is an extractor for the atd files, as well as repacking them.

As for the old move editor which some are after, it's embedded into the game, and can be accessed by typing MOVED in the main menu screen.

Have fun. :)