switch Unity back to Gnome

[Thomas Blasejewicz]

Ubuntu 12.04 LTS

I just did some googling and found, that you can switch Unity back to Gnome.
For that purpose I went (following the instructions) to Ubuntu Software
Center and installed
* Gnome Shell
* Gnome Shell Extensions Preferences

After logging out / or restarting I was supposed to be able to choose
either Unity or Gnome.
Yet, my computer ALWAYS comes up with Unity and does not give me any choice.

Could somebody possibly direct to the setting/operation I missed?

Thank you.
Thomas