If you right-click on a section heading in a Passage Guide, you get a context menu that has an "Add notes" command. At first I was thinking that this command did nothing, because nothing appeared to happen on my screen. But as it turned out, something was happening (a Notes edit box was added to the bottom of that section) but I didn't see it because the section didn't scroll down to show that Notes box. It should.
Screencast: http://www.screencast.com/t/RHN2lxqO