Unable to complete subscription with Authorize.net payment gateway - Membership plugin

I've set up Gravity Forms to work with WPMU's Membership. After the gravity form is filled out, the user is supposed to redirect to the Membership subscription page to complete their subscription by paying for it. (https://premium.wpmudev.org/forums/topic/gravity-forms-and-membership-signup#post-232538)

I'm testing the process and have gotten as far as being redirected to the Membership subscription page. The screen says: "Please check the details of your subscription below and click on the relevant button to complete the subscription."

But there is no button!

The only payment gateway currently set up is Authorize.net. The problem exists whether it is in "sandbox" or "live" mode.