Appointments+ Issues with API creation

Ive installed your Dash. Im trying to go through the steps to attach my calendar and I'm fumbling around a bit because they instructions are dated. I believe ive created the credentials that i need from the Calendar API. I have a Client ID and the Client Secret Key created as a 'Client Id for Web Application'.

The file ive downloaded is a .json, is that correct?

The only purpose of the calendar im using is for my appointments, but its giving me instructions to create a new one. Id rather use the one where all my Google Calendar Appointments already exist. What should I do and how?