For FSX you can doing it using the freeware program Airport Design Editor(ADE), the SDK's object placer, Sbuilderx, or a payware program like Instant Scenery or Airport Facilitator X (ADX). There's probably a few other freeware offerings also but those are the ones that pop into mind right away.
Personally I use Instant Scenery becasue I had it for FS9 and they had a free update for FSX, not to mention that it's easy to use because you can place the object live in game and see the results instantly. With ADE you can also do it and lock ADE to FSX so that you can determine your position that you are going to place the object, however you cant see it being placed live. You will have to save the new ADE file and reboot FSX to see the results.
Just be carefull when placing cars because too many of these little object can start to cause a preformance hit if you get crazy and place too many.
Even though I have the SDK installed I have ever used the object placer so I cant comment on its ease of use.