Have you added any Windows users previously?
Are you trying to add users that are local to the machine that the server is running on, or are you trying to add domain users?
If it's the former then any users that haven't already been added should show up when you click "List server's local users". If it's the latter then it is important that the correct name of the domain is entered and that the domain controller is accessible using this name from the machine on which CompleteFTP is installed. If it isn't, however, then you should get an error saying "The server is not operational" rather than just a blank list.
- Hans (EnterpriseDT)