Well, what are you trying to edit? If you actually want to edit the unit itself (like shape/weapons) then you are going to need a 3D editor. If you just want textures, then look up the .dds-texture converter by Alpaca and just edit the textures in photoshop. Also look up the "How To Add A Unit" tutorial in the Modding Questions forum. It will show you how to use the Model_DB file.

As for your question, you probably forgot to add the faction name to both the sprite line in Model_DB and the faction line. I had a similar experiiance trying to give musketeers to the English.