From what you have said it sounds like the arduino does not sense the output from the plotters hacked led( turned into a digital output) even though you have the 2 usb cables going into your laptop(to complete the circuit).
Take an ohmmeter and measure the resistance between your plotters ground and the arduino ground with and without the usb cables and do not have anything turned on when doing so.
To complete the circuit you need to have both devices having a common ground.