Hi,
Do NOT edit the default files! Create your own and place them in a folder like Addon Scenery/scenery. ADE will find the correct default airport automatically when you use File/Open Airport from Stock (as I remember).
Make sure you are adding the BGL file(s) to a scenery layer equal or higher than any addon scenery you have for the airport.
If you set your compile preferences to split the compile, make sure you are including the OBJ BGL file as well.
Hope this helps,