Welcome to the oDesk Community and thank you for your question. I understand that you would like to pay a freelancer who completed a project for you on oDesk.
Do you have an hourly or fixed-price contract with the freelancer? If it is an hourly, then you will be charged automatically on Monday after the Work Week for hours logged by the freelancer. If it is a fixed-price contract you will have to fund and release the money to the freelancer.
Feel free to browse this category for more information.