I heard about this feature a few months ago, and it seems to have just gone live. Here, users can more easily find airport scenery from third-party developers. http://gateway.x-plane.com/

I tried downloading and installing two airports and found the method to be very simple: just copy the downloaded directory directly into the Custom Scenery folder. There is no need to modify scenery_packs.ini. Below is a screenshot of KAVX airport.

For more details about the Airport Scenery Gateway, please check the official explanation here. http://developer.x-plane.com/?article=airport-scenery-gateway