For the world of me, I don't recall seeing an email (can you tell me who sent it?). I checked the FAQs regarding MiLB.tv and linking it via MLB.tv and saw the following:
Q. I'm trying to watch Minor League games after purchasing an MLB.TV or At Bat subscription, but I can't login or am unable to watch games.
A. You should have received an activation link in an email when you purchased the MLB subscription. Please click the link in that email, and it will first prompt you to create an account for MiLB.com and then automatically grant you access so you can start watching games. If you're unable to locate the email or are having problems, please contact our customer service team or call 866-244-2291, and they can help set you up. (For international callers, please use 315-203-6761.)