You may want to read this thread over http://drupal.org/node/1029002
Ultimately though the answer is summed up at the end

"Posted by PedroKTFC on December 16, 2012 at 6:09pm
Hi Alex

If you go to the product display content type, go to manage display and for the Product field, click on the "gear" link at the right to edit its specific attributes and you'll find the checkbox "Show attribute widgets even if the Add to Cart form only represents one product." there.

Happy clicking!"