[Zope] CatalogAware

I've a problem which I can't solve. I made a small ZClass and has CatalogAware as base class. However, when I add an instance of this ZClass in a folder, and when I've a ZCatalog (id: TestCatalog) then this instance isn't added in the list of 'TestCatalog'. I thought that a CatalogAware Class just could do this... a) Am I missing here something? b) I'm not quite sure, but is this adding to a Catolog done automatically or do you have to specify somewhere in which catalog instance (here TestCatalog) the object has to be listed? How can I do this? Thanks in advance, Tom.