Appointments+ Google Calendar API settings

I follow the 27 step instructions on the plugin settings page for setting up the API correctly via the developer console. When i try to save my settings i get redirected to the frontend?! I have worked out that this is due to this field: "Calendar to be used"

I added "https://www.google.com/calendar/embed?src=laurakerr25%40gmail.com&ctz=Europe/London" because I couldnt find an ID. The intructions are out of date. Where can I find the Google Calendar ID?