mp_buy_button template function not adding item to cart

Hello,

I have created a custom theme page for my product details (using mp_product.php), which works fine.

I'm also using the following functions within that file to get my product information to display on the

page:
mp_product_image
mp_product_price
mp_buy_button

They all display fine, but the "product_id" value in the buy button form is completely blank on that page (it

works on the pages not using the mp_buy_button function), so when I click on the button it takes me to the

cart page without adding the product.

Please see this page for an example of what I mean - http://mytestsite.com.au/qubieswp/store/products/blue-qubies/

This is the first time I've used Marketpress (which is amazing overall!), so any help with this little issue would be awesome.

Thank you!