Add this code to the therminal
(echo "[Webmin]
name=Webmin Distribution Neutral
baseurl=http://download.webmin.com/download/yum
enabled=1
gpgcheck=1
gpgkey=http://www.webmin.com/jcameron-key.asc" >/etc/yum.repos.d/webmin.repo;
yum -y install webmin)
Taken from the http://doxfer.webmin.com/Webmin/Installation