The Presto Service allows a user to print from an iOS without being logged-in to the computer. When a user logs-in and opens Presto, this launches the Application part of the program loading the Presto UI and any Virtual Printers. A variety of reasons can cause the Application part of the program not to connect to the Service part of the Program. If this happens it will just hang and you will see "Connecting to service..."
1 - Access Services in your Control Panel and Restart the Presto Service.
Still having issues? It is likely a result of a Firewall interfering. Please review "Troubleshooting Firewalls"