The path should look like this:
C:\TBP\data\missions\
there you should insert single missions and custom made campaigns which consist of xxxx.fs files(missions)
and xxxx.fc file (campaign file)
You should access custom made campaigns by selecting the appropriate one from the "campaigns menu"
and then "continue" from the main menu
To fly single missions you enter "techroom" than select "mission simulator" and than choose between
current "campaign missions" and "single missions"
This isn't hard to work around

...
Howewer when using campaigns provided by madaboutgames you should follow his guidelines, because they contain sound files and even cutscenes...