When I go to http://mm-ch.wmflabs.org/wiki/Special:CreateMassMessageList currently, there's a disabled source text input field that gets enabled when "Import pages from an existing list or category" is selected.
I think this extra text input is an advanced feature that should follow progressive disclosure principles. Unless the user needs this extra input, we should hide it.
The disabled text input is also terribly difficult to distinguish from an enabled text input in some browsers. Using hide/unhide rather than disabled/enabled would resolve this display issue.
I'm also still a bit unsure about the use of radio buttons, but that's really a separate matter.