For the life of me, I have not been able to figure out how to work the keypad.
I've also searched quite a bit for any mention of how to use it, including the wiki here, with nothing working.
I've tried holding it in both the offhand and main hand, pressing the 1-9 hotbar slot keys, pressing the bound Random PSIdeas spell change keys, both holding and not holding shift, and nothing happens; my spell always says that 'no digit was inputted'.

Coming back here after a while, and I still have no idea how to work the thing. I might just assume it's broken at this point.

No, It's not broken, just confusing.
It refers to the 1st-10th spell keys in the key binds.
You can find them under "Random PSIdeas".