If you're using Chrome but Tone Explorer isn't working, it might be because WEB-MIDI is not turned on.
When you launch Tone Explorer, you should see a keyboard icon next to the Chrome's URL display window, as shown in the orange-highlighted area below. If that icon is overwritten with a slash, it means WEB-MIDI is turned off. In that case, clicking the keyboard icon will remove the slash and turn WEB-MIDI on, so please try that.
fig. 1 WEB-MIDI on
fig. 2 WEB-MIDI off