pushViewController is not working from AppDelegate - iphone

Question

I want to push to a view controller from AppDelegate through an alert view. But its not working. Only the alert view dismisses. Where is the problem? Thanks in advance for the help. (N.B > my initial view is in the storyboard but i am pushing into a view controller nib)
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
loginReapeat = [NSTimer scheduledTimerWithTimeInterval:60.0 target:self selector:#selector(repeatLoginProcess) userInfo:nil repeats:YES];
//First Launch Settings
if ([[NSUserDefaults standardUserDefaults] boolForKey:#"FirstLaunch"])
{
}
else
{
[[NSUserDefaults standardUserDefaults] setBool:YES forKey:#"FirstLaunch"];
[[NSUserDefaults standardUserDefaults] synchronize];
[self alertShow];
}
[window addSubview:[navigationController view]];
[window makeKeyAndVisible];
return YES;
}
-(void)alertShow{
UIAlertView *alert = [[UIAlertView alloc]initWithTitle:#"Help!" message:#"Need some help to use this App? Please tap the 'Help' button." delegate:self cancelButtonTitle:#"Cancel" otherButtonTitles:#"Help",nil];
[alert show];
}
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
NSString *title = [alertView buttonTitleAtIndex:buttonIndex];
if([title isEqualToString:#"Help"])
{
SignUp *signUp = [[SignUp alloc]initWithNibName:#"SignUp" bundle:nil];
[self.navigationController pushViewController:signUp animated:YES];
}
}

Answer 1

Try this:
SignUp *signUp = [[SignUp alloc]initWithNibName:#"SignUp" bundle:nil];
self.navigationController = [[UINavigationController alloc] initWithRootViewController:signUp];
self.window.rootViewController = self.navigationController;

Answer 2

You cannot push from within the App Delegate, instead you can decide which UIViewController to show as a rootviewcontroller depending upon user's selection from the UIAlertView.
self.window.rootViewController = self.navController;

Answer 3

SignUp *signUp = [[SignUp alloc]initWithNibName:#"SignUp" bundle:nil];
UINavigationController *masterNavigationController = [[[UINavigationController alloc] initWithRootViewController:signUp ] autorelease];
self.window.rootViewController = masterNavigationController;

Answer 4

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
self.window = [[[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]] autorelease];
ViewControllerName *home=[[ViewControllerName alloc] initWithNibName:#"ViewControllerName" bundle:nil];
self.window.rootViewController=home;
navcontroller=[[UINavigationController alloc]initWithRootViewController:home];
self.window.rootViewController = self.navController.view;
[self.window makeKeyAndVisible];
return YES;
}