Since you get a new pin code every time you have remaining credits that you have not yet used, it may be easy to lose track of your last (and only good) pin code. In the input box on this page, enter any one of your pin codes that you have a record of and it'll give you the last pin code you were issued.

Probably the easiest pin code to use is the one you received with your paypal verification email.
Your pin code is the shaded area in the following image:

Pin code: