Disregard my post, I figured it out... but I'll leave it here in case someone else encounters the same issue. See my Update at the bottom of this post for how I resolved.
FINALLY found my Tron 2.0 game CD. Whew. Okay, so now I want to try User Error and am having troubles getting it going. I'm trying to skim thru the thread here without having anything spoiled (kinda hard -- I now know there'll be a tough part involving Kernel, and some thoughts about voice acting... etc. Grr). Oh well.
I used REZ Handler 1.3 to unpack the .RAR file and Derezzed (inactivated) two other REZ maps. But I don't have a "Custom" button at the bottom of my game launch window. Am I missing something? I'll keep trying... as I've been able to play custom maps in the past. It's just been a LONG time since I've had to do this, and I don't think it's ever easy, so I hope TronFAQ may come to the rescue.
Here are screens of what I see here:
No "Custom" Button showing:
I figured it out. On a whim, I look at REZ Handler's menus again and saw the "Main" menu option has two options! One to "Install Map
.REZ file(s) from Archive" and one to "Install Mod
.REZ file(s) from Archive". Ahaa!!! A subtle distinction (Map vs. Mod), and one I didn't see in my first attempt, but a very important one. I did the "Mod" one and I now see the "Custom" button. Okay, I think I'll be able to play now. Thanks! Sorry for cluttering up the thread.