Pretty sure its automatic. One of the reasons you didn't get emails too is that with the change all email history went away with the old platform. We all now have to build a new history of subscribed threads etc.
Please check your email address in your profile and make sure it is the one your are actually using. Maybe there is one character off. If that is correct, keep checking here for a reply from a Support Team rep. Wish I could offer more advice, but I'm still in learning mode myself.
Could they be getting stuffed in your spam folder? I don't know if I'm getting emails about new posts on threads I posted in from before the switch, but I'm definitely getting them for threads I've posted in since them.