This is known problem in GNOME Flashback. From terminal:
Code:
gsettings set org.gnome.settings-daemon.plugins.xsettings overrides
'{"Gtk/ShellShowsAppMenu":<0>}'
This will override existing value, by default it is empty. If it has been changed I would suggest to edit it from dconf-editor.
Main problem here is that we are forced to use org.gnome.Shell dbus name. gnome-settings-daemon / unity-settings-daemon when org.gnome.Shell is available sets Gtk/ShellShowsAppMenu to 1 and applications thinks that appmenu will be shown by shell - this is wrong for GNOME Flashback.