Seems that the default theme doesn't have this button very visible, if it has it.There's the Classic YaBB SE Theme that might meet your needs, though.
It's also the one I feel more comfortable to read from, and then it's the one I use.
To change it, go to your profile, then "Look and Layout Preferences"
On "Current Theme" swith to Classic YaBB SE Theme.
Now on the main forum page, you'll have at the bottom of boards a big button at the right that reads "Mark as read".
I guess this is the button that does the functionality you're asking for.
Worth giving it a try.
Edit: It's also on the default theme too. I assumed it was not there, so the question.
Just above the Planeshift Info Center, containing the Recent post and all the stuff, you'll find at the right, the button I'm talking about.