1) First verify that you are licensed for Exchange. Under Help/About on the client you should see that the Exchange module is "Enabled"
2) Verify Exchange has been installed correctly. Check add remove programs for Exchange installation. Also verify you can log into Exchange Management Console.
3) In the Exchange Management Console, verify that your mailbox .edb files are mounted and accessable.
4) Verify that exchange is running the latest service pack. See here for further detail. http://support.microsoft.com/kb/158530
5) If all is correct, try Uninstall/Reinstalling the Novabackup client.
6) If you are stil unable to see the Exchange Database, Open your Windows Services option by typing "services.msc" into run. From the list of services, change the "NovaStor NovaBACKUP Backup/Copy Engine" to run as a local administrator instead of Local System.