appdelegate object in xib - xcode

Question

I am trying to follow the apple photopicker example which has an appdelegate object in an .xib file. How does an appdelegate object get into a xib file, and why is it important?
In my app I want a navigation controller like the photopicker example has. My test work has not included a navigation controller and I am having difficulty inserting one into my test work now that it has just a view controller because the interface builder seems to have made up its mind to not allow the addition. Will it be easiest for me to just start over with a new app that has the navigation controller architecture and then add in copies of my test work methods and classes. Or is starting over unnecessary? I suspect that inserting the appdelegate object into the xib would allow me to use the exist test work more directly.
update 0
I think I followed the instructions in your answer. Xcode suggested _viewController and I changed that. My app is for both iPad and iPhone (and I am working only on iPad now), so it is a little more complicated, I think. I compile fine, but I still see no appdelegate object in my xib file even though PhotoPicker's xib file has one.
I suspect I need to hook up some objects still, but I could use some help with that, too. In PhotoPicker the Connections Inspector for the Navigation Controller says that the Referencing Outlet is between navController and AppDelegate, and the Connections Inspector for the AppDelegate says that the Referencing Outlet is between delegate and File's Owner. But because my xib file has no (BS)AppDelegate object, I don't know how to hook up without it.
#interface BSAppDelegate ()
#property (nonatomic, retain) IBOutlet UINavigationController *navigationController;
#end
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
// Override point for customization after application launch.
if ([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone) {
self.viewController = [[BSViewController alloc] initWithNibName:#"BSViewController_iPhone" bundle:nil];
} else {
self.viewController = [[BSViewController alloc] initWithNibName:#"BSViewController_iPad" bundle:nil];
}
UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:_viewController];
self.window.rootViewController = navigationController;
[self.window makeKeyAndVisible];
return YES;
}
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Answer 1

Inside (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions, you can add uinavigationcontroller like this:
UIViewController *viewController = … //load your first view controller with the initial xib here
UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:viewController];
self.window.rootViewController = navigationController;