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So I can assist you please report the errors you are getting with

<?php
error_reporting(E_ALL);
?>

Post them here and help will come.

http://wpsites.net/best-plugins/free-wordpress-shopping-cart-plugin-increases-sales-at-the-checkout/

This link above should push you in the right area... its called "WP Checkout"

Can you post a tidbit of the code and I may be able to help you!