Hi ForthChaos, I'm glad to hear you're using my Unit Guide.
Now, the DDT is the file assigned to a specific unit that is the texture file, and when converted is the BMP file. You find them when extracting the Textures.bar from the AoM/Textures folder.
Now, if you put the regular DDT file back to the AoM/Textures folder, you will replace the unit in game, but if you are making a brand new unit, you have to rename your DDT file to whatever is the name of your unit in your Proto(x).
Note: If you are using my Creating New Units guide to learn to mod, I would strongly suggest to go through Izzy's Texturing Guide, which is on the same page. It teaches the basics of modding, which lead into making them into new units.
BTW, this is in the wrong forum. Next time try the Modding forum.