Try the next:
Create the file db.php in the root of your site.
Put next code into the file
<?php
// Set flag that this is a parent file
define( '_JEXEC', 1 );
define('JPATH_BASE', dirname(__FILE__) );
define( 'DS', DIRECTORY_SEPARATOR );
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
$db =& JFactory::getDBO();
include('zencart_demo.sql.php');
Put the file zencart_demo.sql.php ito the root directory as well.
Call db.php -
yoursite.com/db.php
P.S. Please backup the database before calling db.php!!!