How to make postcode/zipcode a requied field?

I have been having a rash of customers not entering a postcode in the registration phase and then they are unable to finish an order. Is there a way to make the postcode field required on the registration page in the same manner that name and address are? This would save me lots of time and prevent unknown lost sales.
Version 1.5.6
wattstruckcenter.com/store

It is required already in my default 1.5.6.1 version.
Have you edited any of the account files?

in your Admin -> System -> Localization -> Country -> (select the country you ship to) -> Zip Code or Post Code Required -> Select "Yes"

Cleo