Simply select a forum from the Board Index (the main page) and you will see "Subscribe forum" on the left just above the blue bar, and below "New Topic". Simply click on this, et voila!!
This relates to the sub-silver board layout (the one that you will have unless you have changed it through the User Control Panel). However, if you have a prosilver board layout (like me) you will find the "Subscribe Forum" at the bottom of the page on the left.
You need to ensure your e-mail address is correct - click on "User control panel" at top right of screen, select "Profile" on the left side, then "Edit account settings".