How can I get ARB working in the membership plugin for Authorize.net?

Persuaded client to go w/ WPMU membership so we can migrate existing users who are being billed by Authorize.net – critical to be up and running but no “pay now” button appears to signup. Subscription setting is 10/mo serial every 30 days.

https://thebeta.cloudwaysapps.com/register/?action=registeruser&subscription=3