I imagined that you didn't build it that way... that's why the button is not there. Seems like all the "old" newsreaders (even Windows Mail) does have that functionality and I find it really handy because otherwise I never know what is new since my last visit. There is no such way.
Following threads on Channel 9 is really hard!
1) no way to get notifications
2) hard to understand what is really new since the last visit
3) no way to understand to which thread I replied