In the column on the left, under Others, do you have an option for SSH/Telnet and if so, does it launch? You can then use your default commands to install Joomla or whatever else you want for that matter, providing your logging as root or can sudo.
Or, use the Software Package installer under System/Software Packages.
You may want to look at the Joomla forums, there are several threads involving Webmin, this one in particular:
http://forum.joomla.org/viewtopic.php?p=1468538