It is very easy !
1/ Load the scenery in Landscape Editor.
2/ Zoom to the place you want to add your local airport, and put the cursor of your mouse on the right middle airport place.
4/ Read and write the longitude, the latitude and the altitude of the airport's position, on the bottom line of landscape editor.
5/ Notch Airports on the left panel of Landscape editor (view/modify).
6/ Right click in a void place of the airport's list.
7/ Click onto Add in the pop menu.
8/ Fill the airport data.
9/ Click OK and your airport is added on the airport list.
10/ Flatten to the average altitude all over the airport place.
11/ Export to flightplaner map and save the scenery (menu File)
12/ Fly to watch your airport area, and flatten again if necessary (don't forget to save again)
You can add general feature with Landscape Editor (use generic Files) or make you own airport runways and hangar with Wings 3d or/and with runwaycreator.
That's all folks!
*****- Xavier - (XDL * F-DELX) -*****