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 . . .