The LED is off when you touch the black parts of the screen.
The Slide will send a 'B' to the Arduino; if the Arduino reads a 'B' it turns off the LED.
When the blue button is clicked the Slide will send an 'A' via serial to the Arduino.
If the Arduino reads an 'A' it will turn on the pin 13 LED.
All the code is available on the App Store...