Welcome, Marty!
Some (or all) of this may be obvious, but I'll write it anyway (in case there are others who can benefit):
You'll want to use either Single Shot mode or Frame (Segment) Recording - either one will store the event.
If you use the full 56 M sample size @ 1s/div - you'll get a reasonably detailed view of 14 seconds (7s pre/post trigger). Don't forget to adjust the horizontal trigger position if you want a different division between pre/post-trigger information (e.g. if you only want 1 second pre-trigger, adjust the h.trigger position 6 divs left).
As far as your trigger goes, I assume if the AC goes out, the 15V will go out as well (or vice-versa)? If so, for capture purposes, it doesn't really matter which one initiates the event - as long as you capture enough pre-trigger data to include it. So as long as you calculate the maximum amount of time it takes for an AC dropout to drop the 15V, you can just trigger using a Falling Edge trigger on the 15V. Correct me if I'm wrong with my assumptions - otherwise another option is to use one of the pattern triggers with both channels as sources.
I hope some of this is helpful for you.
(
I also answered your question over in the other main DS2000 thread.)