Now I also searched on google and found a very easy way to solve the problem:
Delete the following file: … documents/[username]/Dreamweaver/Configuration/Menus/menus.xml, start DW (don't click on any menu, this will crash DW!), start Extension Manager, install the extension by clicking on the install button, close Extension Manager, close DW, and everything is ok :-)))
I found it here: showthread.php?t=27049
!!! Please consider the addition in my next post. !!!