Using Arduino/Processing to print text on screen when an LED light up
You don't want your Processing sketch to detect when the LED lights up.  Per your description, you are touching a wire with a wand.  This wire, which I will assume is some kind of switch, has to go into one of the pins on the Arduino that has been configured as an input.  Your Arduino code is then responsible for detecting a change on this input, and then will set the output bit high (or low, if that's how your circuit is set up) to turn on the LED and buzzer.  It is this change in the input that you will also use to make the text appear on the screen.