This FAQ demonstrates how to apply an Auth Code to an LD900 with the use of Quantum.
In Quantum navigate to SETTINGS > System Configuration > Receiver Management and select Receiver Actions:
Within Receiver Actions select Apply New Auth Code:
Enter the Auth Code and click Enter:
Once the Auth Code has been successfully entered, Quantum should be closed, a factory reset should be performed on the LD900 unit and the PC rebooted.