First, click on "My Profile"

Next, scroll down to "Settings & Options"

Next, click on "Edit Options"

To the right of Edit Options section is now "Messaging and Notification"

Within that is "Default Thread Subscription Mode" with a drop down that includes "Instant email notification".

Choose that and don't forget to "save changes" at the bottom of the page.

You should now receive notifications by email when someone posts to a thread you are subscribed to or have posted yourself.