On clicking edit it tries to determine which editor it should use. If
you haven't set any editors, it takes a look in the registry to see
what editors you have installed, and picks the most appropriate one.
If it doesn't find any good editors, it falls back to notepad.
If you go to options, and pick an explicit editor, that will bypass
the search mechanism. My guess is that if you were to install TextPad
(www.textpad.com) then it would start working, since I have TextPad on
all my machines, and may not have tested in the absence of TextPad.