This repository should be enabled by default and is only available for 10.10 and up.
In the Software Center (Applications -> Ubuntu Software Center) navigate to Edit -> Software Sources...
From there navigate to the Other Software tab and ensure the Independent sources are selected
Press close and allow the sources to reload.
Open the APT Sources list
Locate the lines which say
Remove the hash (
#) tag from those lines. If the hashes are not present then that source is already enabled. If those lines aren't there at all you will need to add them to the bottom of the file:
deb http://extras.ubuntu.com/ubuntu precise main deb-src http://extras.ubuntu.com/ubuntu precise main
Close the editor and run
sudo apt-get update
This will download the package cache from the extra's location.