You have a website that that has files on it, and you would like to attach a subdomain to those files.

1)  Log into your control panel.
2)  Click on System > [Domain]
3)  Click on sub domains.
4)  Click on Add New Subdomain
5)  Select "Subdomain on Subfolder"
6)  Type in the subdomain name.
7)  Uncheck the "Create Physical Directory for Subdomain"
8)  Click OK.