Heres a short tutorial I wrote for Bugsyboy a while ago, hopefully its helpful.
1. Uncompress the bg##.pac file 2. Open in Xpacker using Open Pac file with Directories option 3. Extract Pach file 4. Uncompress 190.zlib 5. Open Texture archive 190.uncompressed.zlib 6. Extract all 7. Select all the textures and drag them into Paint.Net (I do this because I dont have a working dds thumbnail viewer, if you know of one let me know) 8. Find the textures you want to edit and open them in Photoshop