This step is about registering the service in your Gateway Hub. Ideally this is done is a HUB system. Service builder gives an option to do this (register service in hub) from the backend system itself without going to the Hub.
To avoid confusion there is a better alternative.
Go to the Hub system itself and register your service as explained here. Activate and Maintain Services - SAP NetWeaver Gateway Foundation (SAP_GWFND) - SAP Library
Even after doing this service builder will not show the service as registered. You may just ignore that.