Well, essentially you have to specify the BGL file and if sending the mision to someone else for something not already present in FSX, also add in the texture skin that covers the object.
Also, you specify the object's location in three dimensions. If the object is a SimObject, such as an airplane, then you can also specify a flight path for it, essentially a series of waypoints complete with altitudes and speeds for aircraft. Ground and water vehicles can also be given waypoints, but altitude is not a factor and only speeds need to be specified. These dynamic objects need to have their own model and configuration files to define their performance margins and limits.
To make this happen, you either need to be very knowledgable about using a text editor and writing up the code, or use a utility like Instant Mission Maker, which I have found very effective. If you have IMM, then you could open up the mission file and be able to simply identify the object and replace it with anything you have in your scenery library.
Ken
