Hi, I'm new to the forums but thought I might be able to help.
I have just installed Webmin on SME Server 6. The best way to install Webmin is using the 'tar.gz' type from the Webmin download page instead of the RPM package. This worked fine for me, simply the follow the instructions further down the 'Downloading and Installing page' on the Webmin site and install it from a shell prompt.
Hope this helps