You can't trace in the perspective view. You need to use an orthographic view, such as the side view. If you are, and it's still not letting you, I have no idea what the issue may be.

One thing you should keep in mind (and is stated in the book's opening section) is that it is not a beginner's guide to maya. I would call my book the second book you read. After reading a beginner's guide to learn Maya basics, then mine would be there to take you into game art from that point.