Just replying to myself... I kept working at it, and finally succeeded. I guess the problem was that the ribbon cable isn't supposed to plug in very far... it goes a little way into the socket, and then the beige plastic clip goes on top and holds it in. Apparently that's how it's designed. I had incorrectly been expecting it to really plug in so it could stay there on its own. Oh well. It seems a little more delicate than it has to be, though...