To be honest, I didn't even know until you asked... I had to go digging to figure it out. But it's in the sub-navigation up top right under the "Forums" tab... "Mark Forums Read"... it will give you an option to mark the entire forum read or just the section you are in. It's going to take some getting used to, but I think it might be better since it allows you to do it from anywhere inside the forum (like inside a thread even).