This dialog box displays a tree view of the available contents within your project group. Expand the project nodes to reveal the nested classes, select the required element, and click OK when ready. 

This dialog box belongs to a general group of selection dialogs where you can choose interactions, operations, ancestor classes, instantiated classes for the objects, etc. This dialog opens when you press the chooser button in a field of the Object Inspector, or when More is selected from the Choose Class or Choose Method menu nodes.