To build a database in P3D you go to DATA, click on the create navigation database file then when the window opens choose

create P3D database
then click on the settings button
go down to P3D.exe and in the dialog box browse to the folder containing the P3D.exe file (mine is D:\P3D V3) and click on the PeD.exe file. that will place that directory in the dialog box.
Below that in the scenery.cfg dialog window go to C:\programdata\Lockheed Martin\P3D V3 and click on the scenery.cfg file.
click ok and that should start the building of your database.

As far as I know the airports marked in red are military airports.

Cheers
Pat

I have a couple of files, one of which is titled NZSI unlisted airstrips.dat. I think I also have one for NZSI as well and several others. Where do these files go in Plan G?