When an application shows a menubar, it is easy to activate items on it by means of alt+<key>. However, if one uses global menu in the title bar, it is currently not possible to do so. One has to use the mouse to open the global menu dropdown, and then one can use accelerators as expected. Perhaps we need to handle this similar to the way firefox handles its menubar, i.e. activate the global menu popup when the alt key alone is pressed.
I have also checked with the global menu plasmoid. Setting a keyboard shortcut to activate it does not give the expected behaviour (i.e., allow to use accelerators). Currently, pressing the keyboard shortcut assigned does nothing.
*** This bug has been marked as a duplicate of bug 376726 ***