Sprite Kit Game Center Leaderboards - sprite-kit

Question

so I've been reading articles and the Apple developer library for a while but I'm not able to solve this.
I want to have a Game Center Leaderboard for my app, that's accessible through my game menu. However, since I'm using Sprite Kit for creating my game this causing me some problems.
I registered my app in iTunes Connect, enabled Game Center and the Identity information within Xcode matches the ones in iTunes Connect.
I created a button, that should open the Game Center Leaderboard. When it's tapped this is called:
GKLeaderboardViewController *leaderboardController = [[GKLeaderboardViewController alloc] init];
if (leaderboardController != NULL)
{
leaderboardController.timeScope = GKLeaderboardTimeScopeAllTime;
leaderboardController.leaderboardDelegate = self;
GKGameCenterViewController *gameCenterController = [[GKGameCenterViewController alloc] init];
if (gameCenterController != nil)
{
gameCenterController.viewState = GKGameCenterViewControllerStateLeaderboards;
UIViewController *vc = self.view.window.rootViewController;
[vc presentViewController: gameCenterController animated: YES completion:nil];
}
}
Now I get the message, that Game Center is unavailable because no player is signed in.
To fix that I tried using the authentication method from apple:
- (void) authenticateLocalPlayer
{
GKLocalPlayer *localPlayer = [GKLocalPlayer localPlayer];
localPlayer.authenticateHandler = ^(UIViewController *viewController, NSError *error){
if (viewController != nil)
{
//showAuthenticationDialogWhenReasonable: is an example method name.
[self showAuthenticationDialogWhenReasonable: viewController];
}
else if (localPlayer.isAuthenticated)
{
//authenticatedPlayer: is an example method name.
[self authenticatedPlayer: localPlayer];
}
else
{
[self disableGameCenter];
}
}];
}
I'm now having trouble defining these methods being called in the authentication method.
My explicit question: How do I authenticate a local player?