I know it’s an old post but in case anyone else has the same issue… I think you’ve moved the WP e-Commerce theme files to your theme folder and should add the code there instead.
You can check by going to Settings -> Store -> Presentation, look for the ‘Advanced Theme Settings’ section which will show if you’ve moved the files.
If you have, edit the wpsc-single_product.php file located at
/wp-content/themes/yourtheme/