Been playin with it, makin custom projects, just cant figure out how to put in an engine so it will go :-/
I'll take a stab...when compiling the model, you need to associate it to an existing aircraft in FlightSimulator in order for it to adopt it's flight characteristics. Best way to do it is to take an aircraft that is close to your project in terms of configuration (# engines, weight, size), and performance. Your aircraft will then fly exactly like the one it has been associated to, and can be tweaked afterward if desired (that can be a project in its own right). FSDS2 and FSDS3 compile differently; the former is very easy and does so "internally", while the latter uses FS2004 MakeMdl (requiring FS2004 SDK as mentioned by Felix above) which you will need to download.