Why don't the airfields appear in the "Search Addon Scenery" list?

This is because, with FSX, we are able to integrate add-on airports directly into the "Search default scenery" list in the Go to Airport dialog. If you search for XSTR (Strathallan) in the default scenery list, you will find Scotflight for FSX's version of that airport, though of course that is not apparent until you actually go to the airport and look round. Note: If you look at your Scotflight for FSX User Guide (there is an icon for this on your desktop), you will find details on the Getting Started page of how to check to ensure that you have the scenery correctly installed.