[Zope] Access to global methods in product creation forms

Hi all. I'm sorry, I've already posted this message on zope-dev, realizing that it was not the right place. I would like to use a global method in the add form of a product. I've declared the method in the methods dictionary of the package. However, when I try to add the product, I receive an unauthorized exception. The environment is python2.3.4 and zope 2.7.3, running on Debian GNU/Linux. I've attached a simple test product showing the problem. Thanks you in advance for your support. Regards Marco