Packet definition files extend the set of event packets that are recognized by the buffer dumper. Chapter 2: Introduction to bufdump concepts describes packets and packet definition files. Chapter 6: Extending bufdump describes how to create packet definition files. Note that you only need to use this if bufdump does not recognize some of the packets that are in your events.
Clicking the -> menu command brings up a simple file chooser dialog. The appearance of this dialog will differ from operating system to operating system. Select the packet definition file you want to read and click the appropriate button on the file chooser