Nevermind, I figured it out; the mod has to be uploaded to the workshop, and you have to make the custom level using the workshop version of the mod.
Thanks anyway, though

Nope, sorry - could never figure it out. Part of why I gave up on the mods I was working on :c

Hey, I know this is unrelated to your map, but I saw your comment on Paril's modding guide, the one that said "I can't seem to select custom levels containing modded weapons. They just don't show up in the list of custom levels." from a year ago. Did you ever figure out the solution to this? I'm experiencing the same problem...