I upgraded to 12.04 right after it was released, and once it installed, I was eager to try the new features. After fiddling around with most of them, I forgot to try the brand new HUD. I tapped the alt key... and nothing happened. I searched around to see if I disabled it, but I didn't. Also, holding alt still showed the appmenu at the top. Is there a reason that it isn't working on my computer?
Could reproduce the problem in a Unity 2D session. When logout and switching to normal Unity session the HUD worked. A source says that HUD is not available in Unity 2D and suggests to add an ppa with enabled Unity 2D.
How do I enable Global Menus and fix HUD (after upgrading to 12.04 from 10.04)? suggests to install
indicator-appmenu. I tried this solution and after a relogin (or
unity --replace in a terminal) HUD was also working in Unity 2D.
There is an open bug report on this issue: https://bugs.launchpad.net/bugs/995040