I have a my views and controllers set up like so.
You can use the follow instruccion:
[(UINavigationController *)self.view.window.rootViewController pushViewController:vc animated:YES];
It works for me :D
A nested ViewController (ie, inside a view controlled by a ViewController that's actually on the NavController stack) doesn't have direct access to the UINavigationController that its parent's view's controller is a stack member of. That's one MOUTHFUL of a sentence, but the sense of it is: you can't get there from here.
Instead you've got to get at the app's NavController via the App delegate.
YourAppDelegate *del = (YourAppDelegate *)[UIApplication sharedApplication].delegate;
[del.navigationController pushViewController:nextViewController animated:YES];
You're using your UIApplication's singleton (contains all sorts of good info about your app), which has a .delegate property pointing to the AppDelegate, and that contains a reference to the NavigationController.
This is how the "Navigation-based Application" Xcode template sets up NavController ownership, anyway. YMMV if you rolled your own--though if you did, you probably wouldn't need to ask this question.
Have a look at UIViewController's navigationController and tabBarController properties. These will return the corresponding navigationController or tabBarController that the given UIViewController 'belongs' to.
So you can do something like:
[customController.navigationController pushViewController:newController animated:YES];
// Similarly for tabBarController ...