Time to debug a little 🙂
Try this:
add_action( 'wpsp_before_content','wpsp_add_custom_meta' );
function wpsp_add_custom_meta()
{
global $wpsp_id;
print_r( $wpsp_id );
$meta = get_post_meta( $wpsp_id, '_your_custom_meta_key', true );
if ( isset( $meta ) && '' !== $meta )
echo $meta;
}Does a number appear?