Note: make sure you are logged in and in the correct event.
This article explains how you can add a participant currently on the waiting list to registered and let him participate in a full event.
Go to Participants, "Waiting" and find the participant you wish to add.
Under Actions, "Change Status" of participant to registered
Find Participant in Registered and Send manually an email to the participant with the request to finalize their registration.
Participant finalizes registration and pays the products and activities required/chosen.