Login as root
cd /root
mkdir webmin
cd webmin
wget http://prdownloads.sourceforge.net/webadmin/webmin-1.580-1.noarch.rpm
rpm -Uvh webmin*
WebMin runs its own web server on port 10000.
To start WebMin, issue this command: /etc/webmin/start.
You access it with a web browser pointed to the IP address of your Asterisk box at that port address, e.g. http://192.168.0.108:10000
You can also manually download the package to your local pc from the links at http://www.webmin.com/
Then using an ssh ftp client, ftp to the centos server and upload the package.
For more good setup info see http://nerdvittles.com/index.php?p=123