How to change master slide title in 6.2?

I have used Keynote occasionally for many years and more or less know what I am doing. I have a strange problem. In a slide master that has "Title Text" at the top, I want to change the words to "Section Title". (Actually, I want to change it to a two-line or two-paragraph title, so I really want "Section Title:¶Topic".) When I select "Title Text" at the top a wide gray-background box shows up. No matter what I try I can't get "into" the text to modify it the way I can with an editable text object on the master. What gives? What kind of thing is this? It isn't even marked as a placeholder. Why can't I change it? (The master is part of the "Classic" theme.)

I know that when I start working on the actual slide, "Title Text" doesn't appear. I also know that if I can type into the title of a slide based on that master and turn it into two lines. The reason I want a two-line title on the master is so I can arrange rules and backgrounds around them. Plus, I'm just frustrated at not being able to change it.