Fatal Error during network activate

Hi!

I just installed Marketpress eCommerce and attempted to activate it on a multisite enabled site, I got the following error, even when activating on the individual site:

Fatal error: Cannot redeclare class MarketPress_Shopping_Cart in /home/content/p3nexnas03/66/2076966/html/wp-content/plugins/marketpress/marketpress.php on line 7793

What does this mean??

Thanks!