Herc du Preez
see comments here: http://www.boardgamegeek.com/filepage/47845/descent-wart-v1-...
Some unzip programs by default unzip without maintaining the directory structure inside the zip file. The directory structure has to be maintained for the program to find the files.
So inside your tileset directory you should have a descent_WART.set file and a descent_WART directory with all the png's and the rest of the .set files.
Hope that helps.