Hi,
Please edit the file administrator/components/com_hikashop/classes/menus.php and change the line:
if($params=='category'&&empty($params)){
to:
if(in_array($params,array('category','manufacturer'))&&empty($params)){
Then, create a new manufacturer menu item. That should create automatically an associated module for your menu in order to display the products listing.
You can also do that manually by creating a new hikashop category listing module via the joomla module manager and then adding that module as associated module in the hikashop options of your menu.