Before a VServer host system can be registered here, you must first bring it under Cloudmin's control using the Add physical system link under Add system on the left menu. It is possible for a single system to both host Virtualmin domains and VServer instances, although this is almost always a bad idea as VServers used for virtual hosting require that the host system not have any server processes accepting connections on all network interfaces.
Once a system is known to Cloudmin, use the Register a system for VServer hosting link to start the registration process.