How to obtain notification about release of new version of Casper Suite?

Personally, I receive an immediate notification from my JAMF account manager.

I suggest that you ask your account manager to be put on a notification list. You can view your account team at:
https://jamfnation.jamfsoftware.com/myAccountTeam.html

Alternatively, try contacting JAMF via their support page:
http://www.jamfsoftware.com/support/

I receive immediate email from my TAM as well. Perhaps you're not registered properly with them as @dmohs states. I'm sure a quick email or phone call will sort you out on that one.

+1 above

Also, if you have a central account for your Casper licensing and then a separate account to post on JAMF Nation, it's going to go to whatever account has the licensing information attached to it. So if you/your org have multiple JAMF Nation accounts you may need to have those other accounts added to the notification list.