I believe that the installer handles things automatically, so you just need to point it to your main FS9 folder, which should be found through a folder string such as this:

D:\Program Files\Microsoft Games\Flight Simulator 9\

The drive letter may be different.

If you want to install the traffic bgl file manually, then that goes in Flight Simulator 9\Scenery\World\Scenery.

The AI aircraft folders go in Flight Simulator 9\Aircraft