I've recently updated SD to work with the coop missions.
You need to have the coop mission map downloaded (as Pilot stated, missions are just maps, nothing else.). Then host a SD game normally, but pick the coop map as the map. Alternatively, pick any map, and then pick a coop map in-game, as normal maps won't work.
Currently the mod will not work for pvp or pvai maps, since this is just a crude test to see if the mod breaks anything important in the coop missions. Once this testing is done, i will re-update it to make it work with normal maps again, and wait for Pilot to put in proper mod support for coop missions.
So don't panic, and play some coop missions with the mod ! If anything breaks, post it here with logs, so i know what to fix before Pilot adds proper mod support.
Thanks in advance!