In customer/main/subcategories.tpl add the red line to the code
{/capture}
{include file="dialog.tpl" title=$current_category.category|regex_replace:"/.*\//":"" content=$smarty.capture.dialog extra="width=100%"}
{ include file="customer/main/navigation.tpl" }</p>
See
http://thecouponclippers.com/docs/customer/home.php?cat=264 for an example.
Hope that helps.
Russ