It's pretty easy. In the case of single missions, simply stick them in Diaspora/Data/Missions.
If there is something other than a .fs2 or .fc2 file in the folder, I'd advise making a folder called 3rd Party Mods with a folder insider it called Data and installing to there instead of Diaspora/Data. Then create a text file inside the 3rd Party Mods folder with notepad called mod.ini and edit the text to this
infotext = 3rd Party Missions for Diaspora;
Then you choose 3rd Party Mods from the Launcher as your active mod.
I think we'll have to include all this with the next Diaspora release. It's more complex than it needs to be for newbies.