[Plugin: Multiple Post Thumbnails] Meta boxes don't show up for products

I’m trying to add Multiple Pst Thumnails to a Product post type. I am using Marketpress by WPMU DEV and I cannot get the post thumbnail meta boxes to show up for products.

It seems that the script never calls the public function thumbnail_meta_box() defined on line 102. I tried adding a echo “test”inside that function and it never gets called.