Go to Events.
Select the Event and View Details.
You can add the attendees to events manually for people that have paid cash, require an invoice or are issued free tickets.
From More Options, select Add Attendees.
Select the Number and Type (if applicable) of Tickets required.
Enter any Custom Field information that may've been required (either per Order or per Ticket). Select Next.
Select the Contact.
Select either Paid or Needs an Invoice (and the due date).
The new booking will appear in the Attendees list with the details of the Tickets purchased/ordered.