iOS 7 sports a new gesture, swiping left to right across the screen as a shortcut for the Back button in a UINavigationController.
My app does not seem to be picking
It should work automatically if the back button is visible. If you are displaying a leftBarButtonItem instead of the back button, the gesture will not be present by default. Also, if you are using a UINavigationBar but not a UINavigationController, you won't see this functionality.
If you are using a UINavigationController and your view controller's navigation item contains a leftBarButtonItem, it's still possible to add functionality for the swipe left to right gesture of the navigation controller, by attaching a delegate to the navigation controller's interactivePopGestureRecognizer.
EDIT: I didn't notice that you're building against SDK 5.1. This is a new feature in the iOS 7 SDK, so I believe you'll need to build with Xcode 5 against the iOS 7 SDK in order to make use of the new feature.