Open the Shadowman folder on your computer and locate this directory:
In that directory, find the files debug.msc and release.msc. Rename release.msc to release.maa. Now rename debug.msc to release.msc. The next time you start a new game, the debug menu should become available.
NOTE: You can restore your original settings by changing the name of your new release.msc back to debug.msc, then renaming the release.maa file to release.msc.
Secret "Play as Duppie":
You must go to the Temple of Life in the wasteland. Get to the offering chamber, the place where you find the Baton, and walk into the 2nd alcove, right to left. As you enter the alcove the secret activated message should appear.
Area 51 mode and Book of Shadows:
You have to have level 10 Shadow Power. When you get level 10 use the bear and warp to the marrow gates. You should come out in the water with 2 waterfalls feeding it. Climb both waterfalls and look for one with a level 10 coffin gate. Open the gate. Inside the gate there is a pedestal containing the Book of Shadows. On the way to the book, the secret activated message should appear. The book of shadows contains concept sketches, comics, drawings, and other cool stuff.
Thanks to Revolution readers Peter Huettner, JaY & KeItH, Yokel Joe and Big Z!