Using loadposition to Show a PayPal Add to Cart Button

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I'd like to put a PayPal Add to Cart button on my product detail pages in place of the VirtueMart Add to Cart button. I have created a Custom HTML module that contains the code for the button. The module is enabled, the position set to ppatcb and the Menu Assignment is set to All.

The Content - Load Module plugin is enabled.

I have placed {loadposition ppatcb} in my /templates/product_details/includes/addtocart_list_multi.tpl.php file. The custom PayPal Add to Cart module is not invoked. Instead I see the {loadposition ppatcb} text on my product detail page. If I put that {loadposition ppatcb} line in an article elsewhere on the site, the PayPal button appears as expected.

What do I have to do to implement the button on the product detail pages?

Ajax = No, SEF = No, VirtueMart 1.1.4, Joomla 1.5.15, PHP 5.3.1