Thought I might as well provide specific installation instructions for anyone using this first beta, even though I know Alex plans on making things quite a bit easier in the future, I suspect there will be at least several more mods released for this early version soon.
navigate to your starfarer installation directory and then from there go to you missions directory:
../starfarer/starfarer-all/data/missions/
now, extract the mission folder to this location so you should have
../starfarer/starfarer-all/data/missions/missionname/
as the new directory of the mission
now just edit
../starfarer/starfarer-all/data/missions/mission_list.json
to include the name of the mission you are adding, and then you should be done!
If starfarer crashes on load, there is probably an issue with the mission name you added, or the creator of the mission did not bug-test their mission, which is really bad form.