Under Software Sources add:
Type: Binary
URI: http://deb.torproject.org/torproject.org
Distribution: lucid
Components: main
Type: Binary
URI: http://deb.torproject.org/torproject.org
Distribution: lucid
Components: main
It should refresh cache when closed.
Search for TOR in software center and install it.
Also install Vidalia.
For More Help:
https://help.ubuntu.com/community/Tor?action=show&redirect=TOR