I rarely touched the Plone 2.5.x these times... 

Catching the module:

In order to handle an module of Zope product, library in Python, the best way is reading its code. The second way, perhaps a little faster when I am on the track of ZMI, is using the nice DocFinderTab (from Jarn). You can file the product here: http://plone.org/products/docfindertab

The product provides clean and clear lists of properties, methods with their description, or parameters,... But when you still cannot find what you need, again, reading the code. Python is really charming when we are doing this.

Catalog notes:

To list out all the indexes registered inside the catalog:

portal_catalog.indexes()

To get all the unique values of a specified field which is already indexed:

portal_catalog.uniqueValuesFor(indexed_field)