У вас есть более одного viewControllers для отображения на вкладке?
Добавьте viewControllers в viewBont viewControllers. USE this property
tabBarController.viewControllers = view_Controllers_Array;
например.
NSMutableArray * viewControllers = [[NSMutableArray alloc]init];
FirstViewController * firstViewController = [[FirstViewController alloc]initWithNibName:@"FirstViewController" bundle:nil];
UINavigationController * nvc = [[UINavigationController alloc] initWithRootViewController:firstViewController];
[firstViewController release];
[viewControllers addObject:nvc];
[nvc release];
SecondViewController * secondViewController = [[SecondViewController alloc]initWithNibName:@"SecondViewController" bundle:nil];
nvc = [[UINavigationController alloc] initWithRootViewController:secondViewController];
[secondViewController release];
[viewControllers addObject:nvc];
[nvc release];
UITabBarController * tabBarController = [[UITabBarController alloc] init];
tabBarController.viewControllers = viewControllers;
[window addSubview:tabBarController.view];