How can an administrator add registration received off-line:

Manage Events --> under the column Manage Registrations click on the appropriate event type (most likely Calendar Event) --> highlight the event you want to register people for and select Manage Attendees --> at the bottom of this screen click on New Registration and your on your way.

Note as an administrator when you get the payment screen you should select "Mail In" payment; keep the check box "Set registration owner to billing information provided below" checked; and enter the registrants information in to the payment screen.  Assuming that you have Financial Account permissions you will be sent to the payment adjustment screen next.  On the payment adjustment screen you can post the off-line payment.