Comment 62 for bug 861268

For me, to switch to Unity-2D (as there is no option in the gears icon when logged in):
* log out
* gear icon: change session to Unity-2d
* log in

And if you need to configure Unity-2d:
http://marianochavero.wordpress.com/2011/10/14/unity-2d-settings-ui-for-ubuntu-11-10-oneiric-ocelot/