Here is the official Doom for Mac demo distribution. It includes the first
episode of Ultimate Doom. The file should be about 3.3MB in BinHex format. Use Stuffit
Expander 3.5.2 to extract it and unstuff it. 

Requires QuickTime 2.0 w/Multimedia Tuner or better, and Sound Manager 3.0 or better. 

Dave Kramer
editor, MacDoom Review
http://www.voicenet.com/~reeltime/mdr.html