Our standard shopping cart will appear as an overlay "inside" your page if you use the standard E-junkie Cart button codes and make sure to include at least one complete block of View Cart code on every page that has any number of Add to Carts.
The page you linked is using a Buy Now button, which bypasses the shopping cart to take the buyer directly to instant checkout for just one item at a time, so that type of button would not use the cart overlay.
I see you're using WordPress, so you may be interested in the WP E-junkie plugin that will make adding our cart buttons to your site even easier while avoiding some common troubles that may happen when attempting to paste our button codes into a WP site manually:
http://wpejunkie.com/
We provide some other tips for WP sites here:
http://www.e-junkie.com/ej/help.site-blog.htm#wordpress
