Conflicting Plugin With Marketpress

When I have Marketpress activated the following plugin does not work http://wp.timersys.com/wordpress-social-invitations/ it tells me in the pop-out that page cannot be accessed directly. You can see the results here http://wun.leadwithanedge.com/wunstore/products/video-scribe by clicking invite some friends. When Marketpress is is isolated or deactivated the social invites work fine.