First ,you are going to need to modify your subcategories.tpl using this mod:
http://forum.x-cart.com/viewtopic.php?t=5131
Next, you'll need to open up customer/main/welcome.tpl and just below this line of code:
{include file="customer/main/featured.tpl" f_products=$f_products}
add this:
{include file="customer/main/subcategories.tpl" cat=$cat}