The schematic is shown above, I've also attached a fritzing document for your reference. This feeder is programmed to manually dispense food when the "set" and "volume" buttons are depressed at the same time. By wiring light sensitive resistors across these buttons we can simulate the mechanical pushbutton action as follows:
When the photoresistors are not exposed to light their resistance will be very high and the processor in the feeder will think that the buttons aren't being pressed. By telling the arduino to light up some LEDs near the photoresistors, the resistance will decrease low enough for the feeder's processor to think that the buttons are being depressed.
I've also attached two pushbuttons in the circuit so that I could manually turn the LEDs on for troubleshooting purposes. These buttons are not essential to the project (but useful).
I also be diverted some power from the feeder's four D batteries to power the arduino.