After you have attended an event you will receive an email to confirm your attendance.
By clicking on 'Yes, I attended' you will land on the event page in the academy. No further action is required, unless you want to change the attendance to 'No'.
In the green part you can read that the manager must still confirm the attendance. By 'manager' we mean the contact person of the event, they must approve the presence of attendees. When they too have done this, the event and your attendance will have the right status.