I'm currently trying to get my own app finished and into the store so documentation/sample writing has been put on hold for a bit - hope to do this one day!
Regarding your issue, this should work fine (it is how I handle this in my own apps). As a first thought have you marked your constructor with the INavigationManager with the [ImportingConstructor] attribute? As far as I am aware MEF will automatically
use parameterless constructors, but if you have parameters you need to explicitly say which one to use when composing.
public class MyViewModel
public MyViewModel(INavigationManager navigationManager)
Let me know how you get on,