How do I get Appointments+ working with Gmail Business Account?

I'm trying to set up Appointments+ with a Gmail Business account and an existing calendar.

Followed the instructions on the page to the letter, but still getting a 403 forbidden error when I attempt to test the connection.

See: http://o7.no/1cKnJcU

Reading through the support forums, it seems like there's a few different suggestions (including contacting google support for some unknown purpose), but nothing definitive.

The only thing I personally noticed was that the format for the calendars with Gmail for Business is in a different format than the free account.

See: http://o7.no/161Qsbm

Do you have any specific instructions for people who are using Google for Business applications or some specific tricks that may resolve our issues?