@chazbotic I've got it figured. Had to open the mission in the Editor, and export it as a .pbo file...

@Soren
for dedicated multiplayer, you should copy the pbo file to the mpmissions directory on your host (or wherever it's configured to read it from) and ensure you have modified your server configuration to either include the mission in the missions class, or have an empty missions class to prompt selecting a mission.

alternately you can self-host and invite people to join you.

I'm gonna feel stupid for asking this, but, what am I moving onto the server to be able to play this MP?