Quote Originally Posted by econ21
Double click the file and it should open in notepad. You can then insert the required lines at the end of the file.

Code:
[misc]

unlock_campaign = true
BTW, one strange thing is that after running the game, those lines then disappear from the .cfg file. But the other factions remain unlocked.
There's that "unlocked.txt" file in the campaign directory. It changes slightly in size when you do this, so I'd imagine it does something to that file, then removes the line from the med.pref.cfg.