The lower C gates (if your flight actually parks there) are separate from C13-16 gates which are miles away from anywhere. It's where United flights used to use and US Airways flights still use. I think it's more straightforward to follow the nearest Exit (passport control) signs and walk to the FCT. When you get down to the baggage claim level you don't have to go outside but walk under roof though you may have to cross a road as C and A/B are separate buildings. It shouldn't take more than 10 minutes.