I know what you're talking about (took a second to twig) but I think you may have the wrong idea.
In game missions are run by players in the Main Arena online, the missions are put together quite simply using the in game editor. They're not like single player flight sim missions that you'd get in a boxed game though and playing them offline would be pointless cos it'd just be you and no-one else fighting....um....no-one else.
If you want that kinda thing then try attending some of the special events run (see the special events forums for details).