There should be a keyboard shortcut to open the xfce-menu. There seems to be a keyboard shortcut for all of the other important things. I've found several online postings of people looking for this functionality in xfce but nobody seems to have found anything
If you want to open the main menu with a shortcut, you should assign the command xfdesktop -menu to a shortcut of your choice. This is in the documentation for the desktop manager, but it's not something one notices right away. I think this shortcut should be defined by default, preferably to the shortcut Control-Escape.
Thank you for the information, and sorry for the invalid bug report.