MarketPress pages not loading. Looks like lightbox isn't loading

I have a multisite installation at http://www.JustThinkBiG.co and I've activated marketpress in one of my sub-site - http://www.ConfidentBusinessWriting.co.uk

Unfortunately, after adding a product, I found that the page would flash up, momentarily, and then what looks like a lightbox appears with a loading 'cog' icon but just doesn't load (see the image, below).

On further investigation, I found that the same happens on all of the marketpress pages. I also found in the settings that the 'Store' page has defaulted to the 'About Peter' page, which shouldn't have happened because I did the automatic page install.

On trying to change the setting to use the 'Store' page that was created by the plugin, I found that the only MarketPress created pages showing in the dropdown menus are the 'Cart' and the 'Checkout' pages. The 'Store', 'Order Status' and 'Product' pages don't appear in the dropdown menu.

I checked the 'All Pages' list to check that all of the pages have been created, are in the list and have been published, which they have.

Please can you let me know how to resolve this issue?

Kindest regards,

Steve