Ahhh ok. I see what you mean. When you click on the add it now link, it brings up the list of pages you can add, and you are supposed to add it from the list. The trouble I believe comes because, the shopping cart is not in the list. It's a feature that was I believe replaced by the social store, when it released, and the shopping cart feature is no longer able to be added to the sites.
Regarding the message you are seeing with the store, it's really hard to tell what may be causing that, without being able to see what all is set up on the site itself, so you'll probably want to submit a support ticket, so that someone is able to go in and take a look at what is going on there.