Implementing Game Center with Sprite Kit - objective-c

Question

I'm unable to implement Game Center into my SpriteKit game.
Most tutorials I've screen require a ViewDidLoad which my game does not have.
I am unable to find correct code to Authenticate my game, or it could be that I have the correct code but am placing it in the wrong area of my application.
Currently I have placed the Authentication in the initWithSize:
#import <GameKit/GameKit.h>
#interface GameScene ()
...
#implementation GameScene
...
-(id)initWithSize:(CGSize)size { /* Setup your scene here */
if (self = [super initWithSize:size]) {
self.anchorPoint = CGPointMake(0.5,0.5);
self.physicsWorld.contactDelegate = self;
...
...
**(I HAVE BEEN PLACING AUTHENTICATE CODE FOUND ON INTERNET HERE, BUT NOTHING FOUND TO BE WORKING)**
}
return self;
}
The below code is where I have a UILabel node called "Leaderboards" where I want my player to be able to click, and enable the user to login to game center and view the leaderboards, along with his own score.
(When clicked currently, it says "Game Center Unavailable, Player not signed in", and then after clicking OK, the user is unable to return to the game, but is left with the game screen frozen?)
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
UITouch *touch = [touches anyObject];
CGPoint location = [touch locationInNode:self];
SKNode *node = [self nodeAtPoint:location];
//if fire button touched, bring the rain
if ([node.name isEqualToString:#"LBButtonNode"]) {
NSLog(#"leader boards opened");
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];
}
}
}
else if(!self.isStarted) {
[self start];
}
else if (self.isGameOver) {
[self clear];
}
else if (self.isStarted) {
[hero jump];
}
}
How do I correctly implement the Game Center feature using SpriteKit?