That will be because DXTBmp is looking for image/tag files, the files in the map directory are .map files. So I assume your wanting to skin something in the game? To do that you'll need to open the map file in either HMT (Halo Modding Tool) or Eschaton, and look in the bitm section for the thing you want to skin. You then need to extract the original skin and THEN open that in DXTBmp and send it to your editting program of choice.