Cart not updating when using Mingle theme and Marketpress

When users are not logged in they can't add anything to the shopping cart. To reiterate, logged in users can add things to the cart.

To test whether this was a theme conflict I switched to the Twenty Thirteen theme and I was able to add things to the shopping cart.

I will be asking Mingle support about as well, but any pointers on what they should look for would be very helpful.

Thanks.