For those who come across this issue of placing product code or any other column in product listing, here is the solution. (Don't ask me how I did it, considering I know zero php)...
Additions in red
<?php
/**
* @package HikaShop for Joomla!
* @version 1.5.0
* @author hikashop.com
* @copyright (C) 2010-2011 HIKARI SOFTWARE. All rights reserved.
* @license GNU/GPLv3
www.gnu.org/licenses/gpl-3.0.html
*/
defined('_JEXEC') or die('Restricted access');
?>
<?php
if(!empty($this->rows)){
if($this->config->get('pagination','bottom')=='top' && $this->params->get('show_limit') && $this->pageInfo->elements->total > $this->params->get('limit')){ ?>
<form action="<?php echo hikashop::completeLink(JRequest::getWord('ctrl').'&task='.JRequest::getWord('task').$this->itemid.'&cid='.reset($this->pageInfo->filter->cid)); ?>" method="post" name="adminForm_<?php echo $this->params->get('main_div_name').$this->category_selected;?>">
<div class="hikashop_products_pagination">
<?php echo $this->pagination->getListFooter(); ?>
<span class="hikashop_results_counter"><?php echo $this->pagination->getResultsCounter(); ?></span>
</div>
<input type="hidden" name="filter_order_<?php echo $this->params->get('main_div_name').$this->category_selected;?>" value="<?php echo $this->pageInfo->filter->order->value; ?>" />
<input type="hidden" name="filter_order_Dir_<?php echo $this->params->get('main_div_name').$this->category_selected;?>" value="<?php echo $this->pageInfo->filter->order->dir; ?>" />
<?php echo JHTML::_( 'form.token' ); ?>
</form>
<?php } ?>
<div class="hikashop_products">
<?php
$columns = 1; ?>
<table class="hikashop_products_table adminlist" cellpadding="1">
<thead>
<tr>
<?php if($this->config->get('thumbnail')){ $columns++; ?>
<th class="hikashop_product_image title" align="center">
<?php echo JText::_( 'HIKA_IMAGE' );?>
</th>
<?php } ?>
<th class="hikashop_product_name title" align="center">
<?php echo JText::_( 'Description' );?>
</th>
<th class="hikashop_product_code title" align="center">
<?php echo JText::_( 'Product Code' );?>
</th>
<?php if($this->params->get('show_price')){ $columns++; ?>
<th class="hikashop_product_price title" align="center">
<?php echo JText::_('PRICE'); ?>
</th>
<?php } ?>
<?php if($this->params->get('add_to_cart')){ $columns++; ?>
<th class="hikashop_product_add_to_cart title" align="center">
</th>
<?php } ?>
</tr>
</thead>
<tfoot>
<tr>
<td colspan="<?php echo $columns; ?>">
<?php if($this->config->get('pagination','bottom')=='bottom' && $this->params->get('show_limit') && $this->pageInfo->elements->total > $this->params->get('limit')){ ?>
<form action="<?php echo hikashop::completeLink(JRequest::getWord('ctrl').'&task='.JRequest::getWord('task').$this->itemid.'&cid='.reset($this->pageInfo->filter->cid)); ?>" method="post" name="adminForm_<?php echo $this->params->get('main_div_name').$this->category_selected;?>">
<div class="hikashop_products_pagination">
<?php echo $this->pagination->getListFooter(); ?>
<span class="hikashop_results_counter"><?php echo $this->pagination->getResultsCounter(); ?></span>
</div>
<input type="hidden" name="filter_order_<?php echo $this->params->get('main_div_name').$this->category_selected;?>" value="<?php echo $this->pageInfo->filter->order->value; ?>" />
<input type="hidden" name="filter_order_Dir_<?php echo $this->params->get('main_div_name').$this->category_selected;?>" value="<?php echo $this->pageInfo->filter->order->dir; ?>" />
<?php echo JHTML::_( 'form.token' ); ?>
</form>
<?php } ?>
</td>
</tr>
</tfoot>
<tbody>
<?php foreach($this->rows as $row){
$this->row =& $row;
$link = hikashop::completeLink('product&task=show&cid='.$this->row->product_id.'&name='.$this->row->alias.$this->itemid.$this->category_pathway); ?>
<tr>
<?php if($this->config->get('thumbnail')){ ?>
<td class="hikashop_product_image_row">
<div style="height:<?php echo $this->image->main_thumbnail_y;?>px;text-align:center;clear:both;" class="hikashop_product_image">
<?php if($this->params->get('link_to_product_page',1)){ ?>
<a href="<?php echo $link;?>" title="<?php echo $this->escape($this->row->product_name); ?>">
<?php }
echo $this->image->display(@$this->row->file_path,false,$this->escape($this->row->file_name));
if($this->params->get('link_to_product_page',1)){ ?>
</a>
<?php } ?>
</div>
</td>
<?php } ?>
<td class="hikashop_product_name_row">
<span class="hikashop_product_name">
<?php if($this->params->get('link_to_product_page',1)){ ?>
<a href="<?php echo $link;?>">
<?php }
echo $this->row->product_name;
if($this->params->get('link_to_product_page',1)){ ?>
</a>
<?php } ?>
</span>
</td>
<td class="hikashop_product_code_row">
<span class="hikashop_product_code">
<?php if($this->params->get('link_to_product_page',1)){ ?>
<a href="<?php echo $link;?>">
<?php }
echo $this->row->product_code;
if($this->params->get('link_to_product_page',1)){ ?>
</a>
<?php } ?>
</span>
</td>
<?php if($this->params->get('show_price')){ ?>
<td class="hikashop_product_price_row">
<?php
$this->setLayout('listing_price');
echo $this->loadTemplate();
?>
</td>
<?php } ?>
<?php if($this->params->get('add_to_cart')){ ?>
<td class="hikashop_product_add_to_cart_row">
<form action="<?php echo hikashop::completeLink('product&task=updatecart'); ?>" method="post" name="hikashop_product_form_<?php echo $this->row->product_id.'_'.$this->params->get('main_div_name'); ?>"><?php
$this->params->set('show_quantity_field', 2);
$this->ajax='';
$this->setLayout('quantity');
echo $this->loadTemplate();
?>
<input type="hidden" name="product_id" value="<?php echo $this->row->product_id; ?>" />
<input type="hidden" name="add" value="1"/>
<input type="hidden" name="ctrl" value="product"/>
<input type="hidden" name="task" value="updatecart"/>
</form>
</td>
<?php } ?>
</tr>
<?php } ?>
</tbody>
</table>
</div>
<?php } ?>