I’ve just tried to add a cart66 item in our demo and it seems to work fine. I have the same WordPress and cart66 version as you have. Do you have the latest version of the theme installed? It should be v4.6. Do you have any other plugins installed?
Regarding the menucard, we have a tutorial available on how to create a menucard: http://www.redfactory.nl/themes/therestaurant/2010/10/18/create-a-menucard/
Make sure to select at least two categories (to make use of the two columns). If it’s still not working, could you post a link to your menucard page so I can take a look?