Well, the answer to that is this:
That option will appear ONLY for active, individual Forums.
When you look at the Nali City Forum Index, what you're really seeing is this:
NaliCity>Visible, but Inactive Forum (acts as Index)
-NaliCity>Visible, active Forum
-User Reviews>Visible, active Forum
-NaliCity - General Forums>Visible, but Inactive Forum (acts as category header)
SUBFORUMS:
--Play The Game>Visible, active Forum
--Other Stuff>Visible, active Forum
--Troubleshooting>Visible, active Forum
--Single Player>Visible, active Forum
-NaliCity - Map Making>Visible, but Inactive Forum (acts as category header)
--Map Ideas>Visible, active Forum
--Play Testing>Visible, active Forum
etc, etc
Only INDIVIDUAL Forums can be marked as read.
What you're asking for is to make an ENTIRE set of Forums marked as read. That would be like having a "Mark as Read" at the bottom of the BeyondUnreal Forums Index.
You can't do it.