Sorry if I'm hijacking the thread but I have the same question.
I got my Plum 96 from Massdrop about a week ago and I wanted to go about adding the LED feature. Problem is I saw no voltage on the few LED solder-points pairs I tried and thus was kinda pissed the marketing line "Just pick your LEDs, solder, and you're good to go" wasn't at all true. Zero-documentation in the box, and none I've come across online either. Even Massdrop ignored my plea to ask the supplier any info regarding the LED placement/orientation/specification/anything?!
So after searching around, seems I feel like this thread is my best bet, seemingly the only one attempting to make this board glow..
Anyway, since OP wasn't able to poke around with his multimeter, I took mine out and documented the continuity, the only thing is that I don't have much experience with creating circuits my soldering experience is limited to repairing solder-joints or splicing wire together. I definitely don't want to solder now. ask questions later, after things get broken so any advice is as to how to get my keyboard lit up is greatly appreciated!
Black = Ground-Ground continuity
Green = Positive-Positive continuity
Blue = GND-to-POS/POS-GND (not sure which way the current flows, diodes confuse me hehe)