Plugin Add-ons installation errors

When I try to install one of the Gravity Forms add ons I am taken to an error page that says, “An unexpected error occurred. Something may be wrong with ww.wp.xz.cn or this server’s configuration. If you continue to have problems, please try the support forums.” So here I am. What can I do to resolve this so I can use the Add ons that I am paying for with that Plugin…

The page I need help with: [log in to see the link]