Depending on what FTP Server you have installed, it will be located under "Servers > ProFTPD Server", "Servers > WU-FTP Server", or something else. If no FTP server is installed or Webmin couldn't find it, it will be under "Un-used modules"
If you're setting up virtual hosting and want automated setup of virtual servers, subdomains, and email addresses, then Virtualmin may be a better for for you. It's a module for webmin plus a whole lot of other stuff which sets up web hosting accounts for you.
If you're looking to manage it yourself with Webmin alone, to set up a subdomain you would:
- Add the DNS A Record for the subdomain to the DNS zone under "Servers > Bind DNS Server"
- Add an Apache VirtualHost under "Servers > Apache Web Server", by using the "Create Virtual Host" tab.
- For the Postfix mail server (you didn't specify what you're using and Postfix is the CentOS default) add a Postfix Virtual Domain under "Servers > Postfix mail server", and add email addresses to that virtual domain.
Virtualmin will do all that and more for you when you click "Create Virtual Server" in Virtualmin. And it will create a username where the owner of the subdomain/virtual server can log in and create email accounts, view stats, etc. It seems like that's what you're wanting. If I misunderstood, please edit your question and clarify.