You can install it with the following command in Terminal:
sudo apt-get install gnome-shell
Afterwards reboot your computer; you can use the following command:
When your Ubuntu loads, just before you log in, click on the Maintenance icon next to your Login, and select Gnome. That will default the Gnome shell for your Ubuntu installation.
Enjoy & remember to use your Superpower button - the Windows key on your keyboard to bring up your Applications bar and Search !