Create your buttons and add a null link. Then create each buttons drop-down menu as a layer. Assign show/hide behaviors to them.
I would guess that the scrolling layers (fixed width and height with a scroller or scroll-arrow) contain hard-coded menu items; not categories dynamically generated.
http://www.projectseven.com has a nice scroller that can be modified any way you wish. They also have free add-ons for Dreamweaver (such as an easy show/hide behavior panel) that will help you get this layout.
Just how I would do it....
Juanita