Any .dds files in the file(s) you've downloaded goes into the Freespace2/data/maps directory. That should get it running in the game. If you want to see it in Fred is a more complicated issue. Since Fred uses ONLY PCX files, and in-game should use ONLY the dds files when using the Earth model.
I can convert the DDS to pcx for you, I'll have it up in an hour. Put the PCX files in the same directory as the DDS's. Once you download them, use it to see what you're doing in fred. After you finish Fredding, get rid of the PCX files, so the game uses only the DDS.