To add Standing Charges click on the 'Add' button which is just below the Standing Charges section on the member detail screen and the following screen will appear:
(this is a subscription charge)
Here you enter the category code from your categories & standard charges. You can also edit the dollar value of the charge if you are charging a different amount to the standard charge. The Reference boxes can be used for trundler or locker numbers - see example below. The 'Stats Date' is added by the system.