Are you sending the email notifications to the same email address as your FormsCentral AdobeID email address? If not, does the email address you are sending notifications to has a FormsCentral account? Make sure that you create a FormsCentral account for that address (can be a free account), accept the Terms if Use and confirm/verify the email address. Only then will you be able to receive notifications to that address.
You need to have a FormsCentral account to be able to opt-in and opt-out of the email notifications.
Let me know if you still have issues