Apple TV 3 and Yamaha RX-V430 AV Receiver

Hi. I need help with my home theatre setup.

I have an Apple TV 3 that is connected to my Samsung TV with HDMI. Now, I want to use my Yamaha RX-V430 AV Receiver to use its 5.1 setup. All I know is that I am going to use the optical audio of my Apple TV 3 going to my receiver. The problem is, which port in my receiver?

Here's the back panel of my receiver by the way

The Optical DVD (1) in the left most part of the back panel is already used.