Make sure that you have both UNE (no longer UNR) and the full Ubuntu Desktop
or GNOME installed. In a terminal type sudo apt-get install ubuntu-desktop
or in Synaptic go to the metapackages section and scroll to the bottom and
install ubuntu-desktop. I am not sure how it is done in Software Centre as I
avoid it as much as possible.

Once it is installed then at login click on the session button before you
login and click on Ubuntu or GNOME. The terminology is changing in Maverick
which is what I am using. It used to say GNOME, but now it says Ubuntu
Desktop Edition or something similar.