Adicionar os repositorios em /etc/apt/sources.list:
http://download.webmin.com/download/repository sarge contrib
deb http://webmin.mirror.somersettechsolutions.co.uk/repository sarge contrib

Fazer download das chaves:
wget http://www.webmin.com/jcameron-key.asc

Reconhecer as chaves
apt-key add jcameron-key.asc

Atualizar o repositorio:
apt-get update

Instalar webmin:
apt-get install webmin