filter custom post type by meta key in dashboard

I’m assuming you’ve used the method in the question linked to and are using the restrict_manage_posts filter.

 add_action( 'restrict_manage_posts', 'my_search_box' );
 function my_search_box() {
     // only add search box on desired custom post_type listings
     global $typenow;
     if ($typenow == 'product') {
       //On custom post type 'product' admin page.
       //Add code for search box here
     }
  }