Doom 3, Steam and user created maps - how to

So, yes, I've been out of it for quite a while - 7 years more or less and I bought the Doom 3 BFG edition not knowing that it uses something called Steam to run off of - which is fine, but how do you play user created maps in a game that isn't even really installed on your pc?

Doom 3 BFG Edition does not support user created maps and mods. Only the original game does.

EDIT: All steam games are installed on your PC. Navigate through Program Files/Steam/steamapps/common and you'll find it there. Still, what I've said above applies.

Wow, that blows. Guess I'll have to go out and get it again because I have NO idea what happened to my copy of it

All those years waiting for the future and the future sucks. lol