Where to connect C wire to?

I just bought an ecobee3 thermostat. My current thermostat is only using 4 wires, but it has an extra unused one (the green one that's wrapped around the cable). My question is where do I connect the green wire to to make it my C wire since I don't have 5 terminals inside my HVAC.

Here are a few pictures including a diagram: Imgur: The most awesome images on the Internet

The unit is 15 years old and I tried to reach out to the company but they switch away from HVAC and can't help.

Thanks in advance!