Thanks for your reactions,
Maarten, yea, I've tried the right clicking method (Copy/Paste) in all possible ways. But Inkscape keeps saying that there is nothing on clipboard. Strange.
Suv, my mac has X11 version ehm.. XQuartz 2.1.6 (xorg-server 1.4.2-apple33)
Is that an old version? Because my mac OS X is updated to version 10.5.8.
Also X11 programs as The Gimp can't work with copy pasting on this way.
I can remember that a few weeks ago I've searched the internet for alt funcionality in Inkscape Mac.
That's working fine now (selecting layers under need layers). Maybe I messed up the X11 settings in order to use the alt button. Is that a possibility?