When you create a Service Definition, it will at first be marked as 'Inactive'. While it's inactive, users will not be able to request it. This means you can get all the settings right before activating it and making it available to your users.
How to Activate
When editing a Service Definition, you'll notice a switch on the left hand side, under the title 'Status':
In order to make the service available to your users, slide the switch across to Active, and Save the changes.
That's it! Any users you have allowed to see this service will be able to request it through the 'Request' tab.
Can I De-activate the Service again?
Of course! If you want to stop users requesting it, or make changes to it, you can simply de-activate the service using the above switch, and Save your changes again.