When a class is accepted by either student or tutor, you will receive an email (as long as your notifications are enabled). The email will be sent with a .ics attachment, which is a widely used calendar format, and is accepted by Google, iCal and Outlook calendars.
At the top of the email, the email service should automatically detect the attachment and present you with an option to add this directly to your calendar:
Alternatively, if it fails to do this you can download the attachment from the bottom of the email:
And then open it with any calendar app that accepts the .ics format