How do I mount my FileVault 2-encrypted volume?
You should be able to unlock or decrypt your FileVault 2-encrypted HD with Disk Utility. I've got a post on how to do this:

Using Disk Utility to unlock or decrypt your FileVault 2-encrypted boot drive:

http://derflounder.wordpress.com/2011/11/23/using-disk-utility-to-unlock-or-decrypt-your-filevault-2-encrypted-boot-drive/

If unlocking via Disk Utility isn't working, you may need to use your recovery key. I've got another post on how you can unlock using your recovery key using the command line:

Unlock or decrypt your FileVault 2-encrypted boot drive from the command line:

http://derflounder.wordpress.com/2011/11/23/using-the-command-line-to-unlock-or-decrypt-your-filevault-2-encrypted-boot-drive/