Ah, well you can't do that type of navigation in the mc shopping cart unfortunately. Think of your site in quadrants, this applies to the default layout of the mc cart.
Top(header)
Left Column
Center Area
Right Column
Bottom (footer)
The areas that will always be static, meaning not changing will be your top, left, right and bottom areas. Which is why your navigation won't work. The only way to get around that break down is to crate a custom cascading javascript navigation that cascades your subcategories out to the right of your main category, like this:
http://www.cddvdking.comIf you are interested in something like that, contact Kyle Prachyl at

Commerce, he is the Design Department Project Manager. kprachyl@monstercommerce.com