When you activate a license on a new cluster and the GUI says it was successful, but you continue to see the Trial Version icon next to the cluster in the SQL Backup GUI you will need to activate through the product activation wizard.

This is installed in your %Program Files%\Red Gate\SQL Backup 10 folder. Alternatively, you can start the program from the SQL Backup Pro GUI; on the Tools menu, select Utilities > Multi server Activation.