SergeA,
You should not have to do anything to AC itself. I just installed the Add-on for AC18 and when I tried to save a 3D view as a C4D file the option did show up as it should. Not sure what your problem might be. It may be sensitive to the type of 3D view you are generating before you try to save it. Sorry not much use.
You will have to wait until someone who knows more chimes in.
Cheers,
Gerald Hoffman
“The simplification of anything is always sensational” GKC
Archicad 4.55 - 27-6000 USA
2019 MacBook Pro-macOS 15.0 (64GB w/ AMD Radeon Pro 5600M GPU)