How do i show modalViewController or navigationController? (iPhone SDK)

I'm trying to show a login page for an app I'm creating but I can开发者_开发问答't seem to get the navigation controller to activate.

- (IBAction)startSignin:(id)sender {
    Login *aview = [[Login alloc] init];
    aview.modalTransitionStyle = UIModalTransitionStyleCrossDissolve;

    [self.navigationController presentModalViewController:aview animated:YES];
    [aview release];
}

How do i get it to show?

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It will never show unless you the view is already added as a subview of UIWindow, which I am assuming is not the case.