The global menu in Natty is provided by the Unity shell, which is a separate thing built on top of Gnome (Gnome2, currently). Therefore, you have a few potential options:
- Install Unity (I'm not sure if it's officially available for Maverick, but I don't see why it wouldn't work)
- Update to Natty
- If you don't like Unity, update to Natty and install Gnome3 from the Gnome team PPA (somewhat risky, as it kills Unity, because Unity currently runs on Gnome2; I haven't had any trouble or need to revert, but if you're not sure you'd like Gnome3, I recommend doing a dry run in VirtualBox)
- If you feel adventurous and can't/don't want to update to Natty, compile Gnome3 from source and install it (probably most risky)
- Find a different shell, or see if there's a plugin of some sort for Gnome2