Appointments+ isn't syncing with Google Calendar

I'm trying to sync the Appointments + plugin with a Google Calendars account. I followed the instructions as closely as possible (Google's interface has changed since they were written), and have uploaded the key to the correct folder and filled in all required fields with information from Google.

When I click "Test Connection", nothing happens. No error message, no success message. But appointments do NOT get added to the calendar when they are added on the site, so it's not working.