PmDefaults is a stand-along application used to set preferences for MIDI input and output devices. The preferences are read by the PortMidi library when it is initialized, and the application can easily open the devices set by PmDefaults. In this way, PortMidi applications can run without any special code to offer device selection. Of course, advanced programs like Audacity can implement their own self-contained selection and preferences, ignoring preferences set by PmDefaults.
When you run PmDefaults, you will see pull-down lists for Input and Output devices reported by your system to PortMidi. If you add or remove a device, for example by plugging/unplugging a USB interface or starting software that creates a virtual port, you might want to press the Refresh Device Lists button to update the choices.
Select the input and output device(s) you wish to use and click the Update Preferences button. You can test devices: input from the selected input device causes the little indicator light to the right of the input selection to light up. To send a note-on to the selected output, press the Test button to the right of your selection.
To set preferences to your current selections and make them visible to PortMidi applications, click the Update Preferences button.
Once preferences are set with Update Preferences, you can exit PmDefaults, and the preferences will remain in effect until you change them. If you leave PmDefaults running, note that ports will be open (see the previous paragraph on testing) and that may prevent an application from opening a device. You should click on the Close/Release Ports button to close the devices held by PmDefaults before you try to open them with another application.