Error message when uploading MarketPress to WP online dashboard

When in my WP dashboard. When I click 'activiate' plug in for my purchased MarketPress. I get the following message:
Fatal error: Cannot redeclare class MarketPress_Shopping_Cart in /hermes/bosweb25a/b504/ipg.3dengravingcom/wp-content/plugins/marketpress/marketpress.php on line 6386

I did try to upload it directly to my file manager as instructed in the wp upload directions.
I have it under wp-content in it's own little folder. In the WPMU DEV it requested my API code, which I placed in the little box.
But when going to Products: Settings in my wp it does NOT have the options for me to add my authorize.net or shipping options.

Please assist.

Do I need to put it into another folder? Like the theme folder?