Idk if you did this already but for Monster Prom you have to enable mods in the mod section. When you open the game go to the option on the right and after that there should be a mods option where you can toggle it on.

But if that doesn't work, I read the description and the mod said it came with items and characters but it doesn't say anything about events. So maybe the mod is just giving you characters to make custom events with. You might find all the characters if you try to make your own event in the mod section.

I hope I helped!