Best luck for modding is by far
http://www.smempire.org.
How to get Arsenal 3 to work:
1. Save your units.dat file
2. Open WinMPQ (you can get this at many SC download sites)
3. Add your new units.dat file to your new .mpq and save it
4. Open MPQdraft and go to create patch
5. Through the screens, make it run on Starcraft and be sure to load your custom MPQ
6. Click done. Starcraft shjould run with your mod on it.