The shuttle flights (missions) are in your Documents/Flight Simulator X files (My Documents/Flight Simulator X files), they are named Entry, TAEM and APPROACH. When you first start FSX in Free Flight, select Load and then look for the one you want to fly (Entry, TAEM or Approach) and away you go.

If you fly the Entry flight, it starts paused. Unpause it and let the flight go on it's way. It's in SLEW mode and I think it pauses at the point where you can take control and fly it. It's just a huge "glider" often referred to as a "flying brick". It is fun, but it isn't easy either.

In the MANUAL Part I – User’s Manual, page 6 gives you some information on the flights.

Hope this helps

Mark