Hi Jens
thanks a lot, your answer led me to the solution. For those interested:
GWT Designer was already successfully installed. What I had to add was Window Builder.
Using the update site
http://dl.google.com/eclipse/inst/d2gwt/latest/3.7 I could choose GWT Designer and Window Builder. GWT Designer was already there so only Window Builder (which was already installed too at that time though...but whatever..) was installed. After that, the missing menu showed up again.
I don't really know what Window Builder has to do with this particular menu for GWT since Window Builder is not GWT-specific and GWT-Designer (which may or may not use Window Builder internally) was already installed, but it works, so I am happy.
regards
Kai