In the editor click make map and select settings and ok. this will save a map.bmp file in the folder marked with what ever your terrain is named.
open this file with photoshop or what ever you use and convert to indexed and save as (terrain name).bmp in the (terrain name)/texsrc folder.
then go back into the editor and biuld terrain and copy and paste the (terrain name).res file into your AH terrain folder.
Hope this was clear enough.