The metronome always sounds to count in the piece. You can also have it play throughout the piece to help you keep time. It clicks on every beat, and makes an accented click on the first beat of each bar.
To practice with the metronome, click on the drop-down arrow on the right-hand side of the Practice button, and choose Practice with metronome. From now on, every time you click on the Practice button it will use the metronome, until you choose Practice without metronome instead.
If you are practicing with the metronome and using a microphone to record your instrument, then make sure you use headphones to listen to the metronome. Otherwise PlayPerfect may confuse the sound of the metronome with the sound of your instrument, with amusing but counter-productive results!