You must install Nvidia driver if you want to use unity.Install appropriate driver form Hardware Drivers.After reboot unity will be loaded.
If you have Nvidia card and you cannot see any drivers for your card, you can use this PPA to install them right away. (this PPA works on 11.04 also) -
sudo add-apt-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update
sudo apt-get install nvidia-current nvidia-current-modaliases nvidia-settings
and restart the system.