OK - this is what to do. As soon as you log on to the system press the New Posts link at the top of the page. Once this has loaded it will show all the threads that have had additions since the last time you where here. To the left of each thread is a little orange arrow
Pressing this will take you to the first unread message in the thread. Although you have to wait for it to load the whole page first. When you have read that message just press New Posts again and it will show you the same list as before but the thread you just read will not ne in bold anymore, indicating you have now read it.