The theme I purchased came without custom fields in the post/product editor. It uses a custom post type in its own admin php file. I've managed to add custom fields by pulling the following code out of wordpress core metabox.php file however I'm unsure how to get it to work. Its missing the area where the custom field values should go.
<div id="postcustomstuff"> <div id="ajax-response"></div> <?php $metadata = has_meta($post->ID); list_meta($metadata); meta_form(); ?> </div>