Right click on your desktop background, if a menu pops up, create a new folder.
Double click on the folder and a nautilus window should pop up. Browse to location /usr/share/applications and find program Compiz, double click on it and magic might happen.
If nothing really happens, go into CompizConfig Settings Manager and check if the Unity Plugin is enabled. If you don't have this program, open a Terminal, and run command:
sudo apt-get install compizconfig-settings-manager or if you dont even have Compiz, run command:
sudo apt-get install compiz
If no menu pops up, go to a tty session, login and run commands:
sudo killall lightdm
If that doesn't seem to work either, check if unity and compiz are installed. Run commands from a tty session:
sudo apt-get install compiz unity
Hope that helps :P