Iphone UI按钮未出现在viewController上

嘿，我想在ipad上呈现一个模态视图，就像这样

login_manager *log = [[login_manager alloc]init];

UINavigationController *nav = [[UINavigationController alloc]initWithRootViewController:log];

nav.modalPresentationStyle = UIModalPresentationFormSheet;

[self presentModalViewController:nav animated:YES];

[log release]`

然后在viewDidLoad中添加一个按钮

-(void)viewDidLoad{

UIButton *facebook = [UIButton buttonWithType:UIButtonTypeCustom];

facebook.frame = CGRectMake(10,10,402,68);

facebook.imageView.image = [UIImage imageNamed:@"loginFB.png"];

[facebook addTarget:self action:@selector(login_FB) forControlEvents:UIControlEventTouchUpInside];

[self.view addSubview:facebook];}

但是我无法让按钮出现在屏幕上，图像链接正确，正在调用viewDidLoad，但没有弹出任何内容。有什么想法吗

(void)viewDidLoad

{

UIButton *facebook = [UIButton buttonWithType:UIButtonTypeCustom];

facebook.frame = CGRectMake(10,10,402,68);

[facebook setImage:[UIImage imageNamed:@"loginFB.png"] forState:UIControllStateNormal];

[facebook addTarget:self action:@selector(login_FB) 

forControlEvents:UIControlEventTouchUpInside];

[self.view addSubview:facebook];

}

-(void)viewDidLoad{    
UIButton *sampleButton = [UIButton buttonWithType:UIButtonTypeCustom];
[sampleButton setFrame:CGRectMake(10, 10, 200, 52)];
[sampleButton setTitle:@"Button Title" forState:UIControlStateNormal];
[sampleButton setBackgroundImage:[[UIImage imageNamed:@"loginFB.png"] stretchableImageWithLeftCapWidth:10.0 topCapHeight:0.0] forState:UIControlStateNormal];
[sampleButton addTarget:self action:@selector(buttonPressed) forControlEvents:UIControlEventTouchUpInside];
[self.view addSubview:sampleButton];
}