Go to 'edit my account' in your user profile (click on your username to open your profile)
Scroll down towards the bottom of the page and look for:
" Notification Method: When you monitor e.g. a forum, how would you like to receive notifications of updates?"
From the pop-up menu, you can choose:
"Email (use address in my profile)"
Scroll down to the bottom of the page and click on "Save Changes"