When you've installed Minion Backup (MB) in a management database, and you try to add that database to an Availability Group, you receive a message that you need to supply the database master key for each encrypted database.
Solution:
By default, Minion Backup creates a database certificate tht you can use to encrypt backups. MinionWare provides a default password in the MB installer, but you're welcome to change it to your needs. If you didn't change the password, then simply go to the MB installer you used and get the password. It's in the line with this text:
"CREATE MASTER KEY ENCRYPTION BY PASSWORD".
If you changed the password when you installed MB, then you should have saved it and there's nothing support can do for you. If you've lost your MB install script, contact support and we'll provide you with the password that was provided with the original installer.
Once you get the password you need, go to this MSDN article for instructions on how to setup your AG with an encrypted database: https://msdn.microsoft.com/en-us/library/hh510178(v=sql.110).aspx