In the Client, make sure the workstation is registered for Host Enabler - Concurrent or Named.
To verify or register the workstation, if you are logged in with administrator privileges, in the Client, select Admin | User Management | Workstation Registration. If the current workstation is the one to be registered, it will be listed at the top of the left side of this screen. Otherwise, scroll down the list of workstations and find the one(s) that need to registered and highlight it.