First up: 3.41 is old. We’re on 3.52 (with 3.53 rolling out as we speak), and we don’t recommend using old versions…we make new versions because they are better, more secure, and less buggy, not (just) because we enjoy building things.
Next, you need to setup Postfix to use a virtual domain map, as the error states.
Browse to the Postfix module in Webmin, click on "Virtual Domains", and fill in the following in the field labeled "Domain mapping lookup tables":
hash:/etc/postfix/virtual
Of course, you haven’t told us what platform and version you are running Virtualmin on, so I’m guessing about file locations…this could be wrong.
The easiest way to get a complete Virtualmin system running is to use the install.sh script on one of the supported platforms (CentOS and RHEL 4/5 and Debian 4.0 at this time for Virtualmin GPL).