Actually, I am the lead Level designer for RETSP, the retsp.wad should be located in the retsp folder, you may freely use the .wad in other mod's as long as you throw us a little credit =) .

If you keep running into problems, feel free to email me at termin8tor2004@ntlworld.com , or to visit our forums at http://www.oraculustudios.com/forums and post in the fan created content section asking for help, make sure you go into detail if your having a specific problem, this way people who have the same problem can seek your thread for the answer they require.

Hope this helps!