From what I figured out it's automatically turned off. I love that the admin's did this cause some forums I go on, the feature is on. When it is on, it sucks sometimes cause your email gets flooded with email you don't need. But that's just me. I only turn it on for really interesting threads.
But if you want to turn it on you have to subscribe to the thread. To do this you have to....
- Click "Thread Tools" at the top of the thread you want to subscribe to
- Click on the 3 option on the drop down menu "Subscribe To Thread"
- You will be taken to another page where you can select how frequent you would like to receive an email notification. It can be Instant, daily, or weekly.