Mailman is a Virtualmin plugin. I don’t think it provides a Webmin UI. You use it as a feature in Virtualmin virtual servers.
You’ll need to install Mailman itself, also.
Once Mailman is installed, you can run virtualmin config-system --include Mailman to maybe do the basic setup (I don’t know if this works with a newer Mailman, though, I’ve not tested it, you may need to manually get the Mailman service up and running…this used to be kind of a PITA, as the service won’t work at all until you create the first mailing list for administrative purposes).
Then enable it in Features and Plugins, and you’ll be able to create Mailman mailing lists for your domains and manage the lists from the UI. There will be a menu item in the Virtual Server section of Virtualmin for it.