I would like to install webmin, but it is neither available from Synaptic, nor from
Why can't I find it in synaptic or apt-get? How can I get it and install?
Webmin is not in any Ubuntu repositories. You will need to download the debian package file (.deb) at http://www.webmin.com/download.html
Their installation instructions for Debian apply to Ubuntu (and include Ubuntu specific notes) at http://www.webmin.com/deb.html