The only file you need to get the editor up and running is DromEd.zip.
You need to unzip it in a pre-existing Thief Gold install directory 
(for example C:\games\thiefg - or wherever you have installed Thief 
Gold).  When you unzip dromed.zip, you will be asked if you want to 
overwrite some files, answer "Yes to all". This will insure that all of 
your config files are set up right.  You will need a Thief Gold CD to get 
all of the resources - the art, sound, mesh & mocap files.  You will also
find a new subdirectory in your Thief Gold directory called "docs". A
tutorial and some other DromEd reference files can be found there.

If you want to see some examples of pre-existing levels or you want to 
change one of the levels that shipped with Thief Gold you can unzip the
relevant mission files in this directory.  Please not that these are
special versions of the mission files.  When we shipped Thief Gold we
"stripped" some information that only the editor uses out of the mission
files in order to make them smaller and save space on the CD and your hard
drive.  If you want to edit a mission file from Thief Gold you have to use
one of the files in this directory.  

It is also important to note that you should create a different directory
from your Thief Gold directory in which to place the "unstripped" mission
files.  If you unzip them in your Thief Gold directory you may experience
problems with potential future patches.

Mission Listing:
miss1.zip - Training Mission
miss2.zip - Lord Bafford's Scepter
miss3.zip - Break from Cragscleft Prison
miss4.zip - Down in the Bonehoard
miss5.zip - Assassins
miss6.zip - The Sword
miss7.zip - The Haunted Cathedral
miss9.zip - The Lost City
miss10.zip - Undercover
miss11.zip - Return to the Cathedral
miss12.zip - Escape!
miss13.zip - Strange Bedfellows
miss14.zip - Into the Maw
miss15.zip - Thieves' Guild
miss16.zip - Mage Towers
miss17.zip - Song of the Caverns
miss18.zip - Outtakes