Objective-C & Interface Builder for Dummies: How mix different controllers and transition between them?

I want to make an iPad application (I'm actually making it now, it's just not working...) where I start out with a login screen for the users and if they authenticate I want to transition to another screen which will have a TabBar.

Right now I have my iPadAppDelegate with a MainWindow XIB file. In MainWindow I have a SignInViewController which is matching a class and XIB file of the same names. The SignInViewControlelr XIB contains all the text fields and buttons the user needs to sign in.

If the user is authorized I want to transition to the screen with a TabBar, which is why I have a UITabBarController in MainWindow XIB, but I can't seem to transition to it.

Ultimatelly, I think I'm misunderstanding how Cocoa's version of MVC works (my MVC knowledge is limited to ASP.NET MVC since it was in v1 preview 1... And as I can see it right now there's big differences, but I am most likely very wrong, hence the misunderstanding). Perhaps I'm using too many controllers and too little views or just not placing them where they should be, let alone writing the code to interact with them...

I'd appreciate an explanation on how I should properly structure the controllers and views to get what I want to achieve. An app similar to what I want to do is the AT&T myWirelss app.

Help would be appreciated!

P.S. I've litereally been learning how to make iOS apps from watching the videos of xvitcoder on YouTube and further adding the functionality I need while asking questions on how to add the functionality I need. The videos are good (I think), but I'm trying to do something that isn't really explained by them (I think).

UPDATE

The code I ended up using, which works, although I'm not sure if its the proper way of doing it:

DashboardViewController *dashboardViewController = [[DashboardViewController alloc] initWithNibName:@"DashboardViewController" bundle:nil];
[appDelegate.window insertSubview:dashboardViewController.view aboveSubview:self.view];
[dashboardViewController release];

You may find the View Controller Programming Guide informative. Ultimately you can have a derivative of UIViewController that you use to internally manage your different views/view controllers or you may also utilize a UINavigationController.

A really good simple model of flipping between big views is provided by the "Utility Application" of the iPhone templates. It shows how a view controller can call another controller. This might give you a start.