After configuring Exchange 2013 DAG database and configuring the database backup in Netbackup, it would fail with error 2.
After a lot of reading, many a trial and error, I found this little gem http://www.symantec.com/connect/forums/exchange-2013-error-2
In your policy under Microsoft Exchange Attributes, Netbackup wants the preferred server list populated.
Choose your Source – Active or Passive. Click Preferred Server list and add your DAG nodes.
My Working Configuration
Backup Selections: Microsoft Exchange Database Availability Groups:\DAGName\Microsoft Information Store
Snapshot Client “Perform Snapshot Backups”
Microsoft Exchange Attributes: Source. Populated Preffered Server list with DAG nodes.
Clients: YourDAGName (Cluster DNS name for your DAG)