HI there
I have been struggling to do the same
Perhaps the easiest way to explain how to do it - is to show the example for one created by the custom data creator
Click on Setup, custom data, create custom data
Then to create an airfield click on airfields
then left main file column click New file (in my case tocreate then new one of UK)
Thrne in the right column click pn new and up will pop the airfield editor
and pop in the data and coordinates
I also used google earth to find the coordinates to use
I dragged an untitled placemark onto one end of the airstrip and then copied and pasted in the coordinates and then removed the unneeded symbols and adjusted the format to match what skydemon likes.
Then I did the same for the other end of the strip and Magic an nice skydemon airstrip
I just wish I could understand how to get the circle and polygon functions to work and then the world would truly be my oyster!
Tom