You need to enter anything into the serial monitor and hit send. It doesn't matter what you type, I usually type "1". That should cause the program to start outputting values. The default output is for the teapot processing program to use (i've never used it so i can't vouch that it works, but I assume it does). It will look like gibberish and not mean anything to you. There are several #define commands near the top-middle of the code that if you uncomment will output things like Quaternion, Euler Angle, Yaw/Pitch/Roll, Real-World Acceleration and Readable Acceleration (gravity compensated) values in the serial monitor instead.