Hello Mrinal,
You might want to refer to the following article for your queries related to the CAS/MMS feature in Backup Exec 2012:
1. Best practices for Backup Exec 2012 Central Admin Server Option:
http://www.symantec.com/docs/HOWTO74432
2. Upgrading to Central Administration Server Option 2012:
http://www.symantec.com/docs/TECH178278
As far as downloading the application is concerned, all you need to do is to add the Enterprise Server Option(ESO) option to your existing Backup Exec 2012 installation and it would enable the CAS/MMS feature.