Hi Lars,
my bad. It’s Windows-sh… using Apple Software with a bad implementation (iTunes)!!! 
When choosing the “Cubasis LE Documents” there are the 7 folders contained I mentioned above. I still cannot open the folders or drag and drop them although the icon suggests it is doing so.
On my screen the rows below the folders are all empty down to the bottom of the screen (at least 25 of them). I did not see the button you mentioned up to now. After reading your latest comment I realized that there’s a scroll bar at the right side of the iTunes Window although there is no content below the shown files (really not following Windows UI and usability standards!!!).
I scrolled down and suddenly I saw the two disabled (greyed out) buttons at the bottom of the screen - but not the folders anymore.
I scrolled up again to the top, selected the “Mixdown” folder, scrolled down again. The “Save…” Button was still greyed out, but when I clicked it, it opened a “Save As…” dialogue window and I could save the Mixdown to the Laptop. SUCCESS!!! 
I absolutely excuse myself for bothering you with this APPLE ISSUE and stealing your time!!!
Steinberg did the things right and I made you suffering the bad implementation by Apple on Windows with me! Sorry for that! I excuse myself and beg your pardon!
Now I can use and edit my files and project and continue to work with it in Cubase AI using the Cubasis Project Importer and in other tools.
Thanks a lot for your great support and patience with me! Best regards,
Karsten