Oh sorry, I misunderstood your post...I thought you wanted to put the description in the SIDE menu!
To do what you want, just edit subcategories.tpl and replace this:
Code:
{ include file="customer/main/navigation.tpl" }
{if ($navigation_page eq "")||($navigation_page eq "1")}{$current_category.description|regex_replace:"/[\n]/":"
"}
{/if}
{capture name=dialog}
{assign var="tmp" value="0"}
with this:
Code:
{capture name=dialog}
{ include file="customer/main/navigation.tpl" }
{if ($navigation_page eq "")||($navigation_page eq "1")}{$current_category.description|regex_replace:"/[\n]/":"
"}
{/if}
{assign var="tmp" value="0"}