The Article tab works differently than all other tabs in vBulletin. You cannot assign arbitrary links to the sub-menu.
In the CMS use the pencil icon to edit a section.
There is a block at the bottom of the page called: Sections to Display in Navbar Sub-Menu
Here you can show which CMS pages you can show in the sub-links.
There is also a checkbox in the block that allows you to select what CMS links you want for a designated "sub-link."
Once you fool around with it a bit and create your own pages you will get a feel for it. It is a bit difficult to comprehend at first.
I hope this at least gets you started.