How to add "Add to Cart" Button on items list instead of "Buy Now"

I need to be able to have "Add to Cart" under an item thumb in grid view on a global list, where it will use ajax style to add and refresh the cart widget. This is the main site that I have problem with. Child sites is ok.

In marketpress.php, I tried to change below in function framemarket_mp_list_global_products
--------------
//price
$content .= $price;

//button
//change below to add a Add to cart instead of Buy now (which goes to product page
$content .= '' . __('Buy Now »', 'mp') . '';
---------
to
---
//price
$meta = mp_product_price(false, $post->ID, false);
//button
$meta .= mp_buy_button(false, 'list', $post->ID);
$content .= apply_filters( 'mp_product_list_meta', $meta, $post->ID );
---
but it just doesn't add item to cart, neither refresh the cart widget.
So i change it back and you can see it at http://submittedbyme.com/store/ for the main store; and http://youparkrudely.com/store/ for a child store.

Thanks so much for your help.

Ken