Hans
Sorry for the delay in replying to your question.
So first thing...create a folder when you would like your user installed aircraft to reside. I would suggest one outside of the main P3D folder.
Start the Addon Manager, select "Other Addons" with the "Simobjects" category selected from the drop down menu, in the "path" point it toward the folder you created, enter a name, description in the "add-on.xml data" section before populating the category area with similar information.
Hopefully the image below will help also.
Add a aircraft to the new folder and all being well it will be available in P3D. This method works for freeware aircraft. I have not tried it with payware aircraft.