Thx for your help Oskar, I de-installed Fuerte, started P3d , started dbBuilder...Warning is gone
Installed Fuerte again, started P3D (no Add-on.xml message), started dbBuilder....still no warning
So everything is fine now although I still don't understand it as Fuerte is definitely installed using the old method.
But anyway...result is as wanted, thank you very much!
Klaus