Hi glv,
Thank you for your answer! You are right about commenting Serial.println() on Arduino.
However, the buttons still not work. The LED is blinking when I run the Processing sketch, but when I hit the Z button, nothing is happening with the LED.
I tried it out with if and else if, insted of a switch case in my Arduino code, and that one is working with Processing as well. I am just wondering what could be the difference.