Well, you can't add a VC to an aircraft.  It just isn't possible, because the VC is part of the model and is compiled by the author.

To put a VC in an already existing model, you would first have to know how to use a 3D design program (FSDS, Gmax), then you would have to get the source files the model was created from off the author.  Then you could modify them and create a VC.  All that sounds waaaayyyyy too easy for what's actually involved . . .

As for modifying the scenery, I would suggest either EZ Scenery or Runway 12, which will both allow you to place objects in FS by "hovering" over the spot in slew mode and then pressing a button on your keyboard.  To remove objects, you can add a flatten command in the scenery.cfg in the main FS folder.  Can't remember how to do this, exactly.  Hopefully some other nutter around here can . . .