A number of people have commented that they have clicked the link to bookmark a thread but never received an email notification when that thread was updated. This took me a little while to figure out, but the problem is that in the ProBoards system email notifications are off by default.
To set this up, click the 'profile' button in the top menu bar, and then click the 'bookmarks' link found to the right of the modify profile link. If you currently don't have any topics bookmarked, then click the link that says "Click here to manage your bookmarks preferences.", otherwise click the 'Manage Bookmarks' link found in the top right hand corner of the bookmarks listing table.
On the next page, you'll see the following: "By default, when you bookmark a thread, what type of notification would you like to receive when a new post is made in that thread?" -- set this to 'Email' and then click the update bookmark preferences button. After this has been set, any thread that you bookmark using the button within a thread, you will receive email updates for.