I tried the following way but failed.
Under /includes/templates/template_default/common in tpl_header.php file, I added the third line: <li><a href="http://your_main_page_url">Main Site</a></li>
<div id="navMain">
<ul class="back">
<li><a href="http://your_main_page_url">Main Site</a></li>
<li><?php echo '<a href="' . HTTP_SERVER . DIR_WS_CATALOG . '">'; ?><?php echo HEADER_TITLE_CATALOG; ?></a></li>
<?php if ($_SESSION['customer_id']) { ?>
<li><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGOFF; ?></a></li>
<li><a href="<?php echo zen_href_link(FILENAME_ACCOUNT, '', 'SSL'); ?>"><?php echo HEADER_TITLE_MY_ACCOUNT; ?></a></li>
On clicking the logo, I still get the shopping cart page (directory) instead of home page.