When you hit Start, notes will be displayed in the stave above. Just use the mini claviature below to play the notes. Don't worry about the rhythm or speed for now.
The generated notes will be so that you play only one note at a time. If you want to practice chords, have a look into the Set Up section to hook up your digital piano.
When you start the training, we will count in for 4 beats and afterwards you can tap the given rhythm (either use your ‘space’ button or your touchscreen). Make sure your speakers are on so that you can hear the metronome.