Hi,
This does work actually, but the design is so poor you can't see it.. !
Unread posts have a red document icon in front of them, read posts have a black one. It seems during migration everything was marked unread. You can mark all as read if you click on the "Not read" link at the top which brings you to all undread posts, where there is a "Mark all read" button.
We'll see what we can do to improve this.