In the window entitled "MIDI Sources,"
the MIDI devices connected with the computer are listed.
In the default, only the first device is available from MIDI AE.
You can change the usage for each device by check on and off the
checkbox at the left side of the list.
If you turn on or off, connect or disconnect MIDI devices,
you can revise the device list by clicking "Refresh" button
at the bottom of the window.