Run the command:
whereis webmin or
rpm -qa | grep webmin to see if it is installed.
Webmin is a service and that needs to be running to use. Check to see if it's running with the command:
service webmin status
If it needs to be started, run the command:
service webmin start
If you want it to run every time you reboot the computer, use chkconfig...
chkconfig --level 5 webmin on
To actually run webmin, use the konqueror ( or firefox .... ) address bar.
For example:
http://localhost:10000
or
https://localhost:10000