Error when purchasing download object with marketpress

“After having successfully completed a test purchase using Marketpress and Paypal, I received the following error message:

Warning: MarketPress was unable to download the file for serving as download: Not Found in wp-content/plugins/marketpress/includes/public/class-mp-public.php on line 707

Whoops, there was a problem loading up this file for your download. Please contact us for help.”

I don’t know what this means or why it would show up.