Midi in is not supported yet, i.e. you can't press a key on your connected MIDI keyboard to send a MIDI message to the worklet yet. You can simulate it with the keyboard on the left. Quoting the Help page (accessible with the "?" button on the top right):
"The MIDI IN keyboard will send MIDI messages to the worklet's MessagePort. Messages are objects of the form {type: ['noteon' | 'noteoff'], value: { note, octave, midi, frequency } }"
Using physical MIDI controls will be supported in future releases, though.