redirect loop when membership plugin activated

Hello, I am building a marketpress multisite, and when I activate a WMPU Membership plugin, I cannot access the marketpress shopping-cart page and order-status page as a visitor. I get a message saying that the page isn't redirecting properly. I tried to change the access level, but that won't help. When I deactivate the Membership plugin, I can access the shopping-cart page without having to log in. I would like to have the membership plugin and have customers access to those pages without logging in. How can I fix this problem? Please help me.