Hi Holger.
If you have installed the P3D SDK, you can navigate to this folder and copy your XML file there: ...\Prepar3D v5 SDK 5.0.31.35253\World\Scenery
Then drag your XML file onto the bglcomp.exe file and you will receive a BGL file
If it has to go quickly and I need coordinates, then I do it in Google Maps. To do this, click on the map with the left mouse button on the point where the effect should appear and the coordinates in the decimal system appear at the bottom:
If you now click on these coordinates, they will appear on the left side of the browser ...
... from there they can easily be copied ;-)