Admit a waitlisted attendee

• Event → Registration→Waitlist
• Select the attendee(s) you'd like to move off of the waitlist by checking the box in front of their name
• Choose Attendee Status→ Confirmed→ Update 

This will switch the attendees into a Confirmed / Active status. It does not automatically email them. You will want to do that to let them know they have been accepted into the event.