Game Center not showing in iOS 5 - objective-c

Question

I used this tutorial to access GameCenter.
http://www.raywenderlich.com/3276/how-to-make-a-simple-multiplayer-game-with-game-center-tutorial-part-12
I am able to lo in the game center but then if I want to use this code:
SFAppDelegate * delegate = (SFAppDelegate *) [UIApplication sharedApplication].delegate;
[[GCHelper sharedInstance] findMatchWithMinPlayers:2 maxPlayers:2 viewController:self delegate:self];
This code should work, but if I call i it only shows black screen with blue toolbar in above.
the "self" in here is UIViewController and this controller implements the GCHelper delegate methods.
the method find ... looks like this:
- (void)findMatchWithMinPlayers:(int)minPlayers maxPlayers:(int)maxPlayers viewController:(UIViewController *)viewController delegate:(id<GCHelperDelegate>)theDelegate {
if (!gameCenterAvailable) return;
matchStarted = NO;
self.match = nil;
self.presentingViewController = viewController; // ?
delegate = theDelegate;
[presentingViewController dismissModalViewControllerAnimated:NO];
if (pendingInvite != nil)
{
[presentingViewController dismissModalViewControllerAnimated:NO];
GKMatchmakerViewController *mmvc = [[[GKMatchmakerViewController alloc] initWithInvite:pendingInvite] autorelease];
mmvc.matchmakerDelegate = self;
[presentingViewController presentModalViewController:mmvc animated:YES];
self.pendingInvite = nil;
self.pendingPlayersToInvite = nil;
}
else
{
[presentingViewController dismissModalViewControllerAnimated:NO];
GKMatchRequest *request = [[[GKMatchRequest alloc] init] autorelease];
request.minPlayers = minPlayers;
request.maxPlayers = maxPlayers;
request.playersToInvite = pendingPlayersToInvite;
GKMatchmakerViewController *mmvc = [[[GKMatchmakerViewController alloc] initWithMatchRequest:request] autorelease];
mmvc.matchmakerDelegate = self;
[presentingViewController presentModalViewController:mmvc animated:YES];
self.pendingInvite = nil;
self.pendingPlayersToInvite = nil;
}
}
I don't know what am I doing wrong....
Thanks for any suggestions for correction.
:)