If you unregister a virtual machine from a host, but you do not delete the virtual machine from the datastore, you can re-register the virtual machine by using the VMware Host Client. Re-registering a virtual machine makes it appear in the inventory.
Use the datastore browser to select either a datastore, a directory, or a .vmx file to add to the list of virtual machines that you register. Selecting a datastore or directory searches for all .vmx files in that location. You can browse more than once to append virtual machines to the list.