Unfortunately Ubuntu liNUX does not offer web based administration directly, instead users have to manually add the Debian repository. This is inconvenient and its a fair amount of work involved to add this to the system. Now you need to install webmin from APT repository

sudo gedit /etc/apt/sources.list

then add to the bottom

deb http://download.webmin.com/download/repository/ sarge contrib

sudo cd /root

wget http://www.webmin.com/jcameron-key.asc

apt-key add jcameron-key.asc

apt-get update

apt-get install webmin