The 2010 forum post https://www-secure.symantec.com/connect/forums/veritas-netbackup-mysql-db is no longer relevant as support has in the meantime been added for MySQL.
See NetBackup 7 Application/Database Agent (CL): http://www.symantec.com/docs/TECH126904
As per the CL, you need to contact Zmanda and purchase the agent directly from them.
Seems you don't need a NetBackup Application and Database Pack license, as it only covers agents for the following databases and applications:
Oracle
Microsoft SQL Server, Microsoft Exchange Server, Microsoft SharePoint, Microsoft Active Directory
IBM DB2, IBM Lotus Notes
Informix
Sybase
Symantec Enterprise Vault