The concept of app.config files is not part of Windows Store App development. The docs and postings say that one should use local or roaming settings. This sample application copies the appSettings part of .NET app.config usage. It is also able to handle more complex data structures, not only key / value pairs, by using XML serialization.

The app also contains the source code. This enables you to build and test the app on your own machine. And in case you like the approach, you can use it for your own apps.
But think twice before using it. Don’t use it just because you are used to it from .NET applications.

If you have the need to save configuration data during runtime, the app.config mimic is not the right approach to me. Better use local or roaming application data then.