I finally got the plugin working once I discovered the conflict was with 404 Redirect.
Now when I attempt to buy a product by clicking Buy Now button...it does not add the item immediately to the cart.
Instead it goes to the Shopping Cart page
/shopping-cart where there is a "Browse Products" link. I then have to select a product and click the Buy Now button again!
I am using the shortcode [mp_product product_id="2" image="0"]
I need the Buy Now button presented by the
[mp_product product_id="2" image="0"] shortcode to add the product to the Shopping Cart immediately, not having to select it again on the Products page.