I just installed MarketPress and was looking around to familiarize myself with it.
I was in the admin orders screen and clicked "Add New" There are no orders at this point. Only one test product for $25.00. I got the following message:
Fatal error: Call to a member function is_download_only() on boolean in /home/user/mydomain.com/wp-content/plugins/marketpress/includes/admin/class-mp-orders-admin.php on line 307