I found it. The buttons used to be at the bottom of the screen, but they've now (I don't know when this was done) been moved to the top of the window where the song playing and the progress bar are displayed. They become visible when you play a song, and the button you need is the one on the left. Incidentally, while looking for this, I found that I'm not the only way who hates the latest versions of iTunes. I found a page where everyone shared my belief that it was one of Apple's bigest mistakes.