I've had lots of problems with playing mods so you're not missing out on much lol. The mod content is broken for me, I keep getting an error that "you don't have the host's map". So I gave up. At least, I give up until they patch it.
If this is on the GOG side, that'd be because there aren't any multiplayer maps out for that version yet so far as I'm aware. If you're trying to port maps from the Steam copy of the game I would guess that you'd need to preserve the name of the folder the map is in and paste it to the /mods directory, but I have yet to try that myself.