It would be better to add another link versus one that takes them out of your cart...

But that is the line for your link on the logo

Try editing this in a copy for your template to
Code:
<div id="logo"><?php echo '<a href="Your_link_here">' . zen_image($template->get_template_dir(HEADER_LOGO_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . HEADER_LOGO_IMAGE, HEADER_ALT_TEXT) . '</a>'; ?></div>