I spent hours trying to debug this one so I thought I'd post here to help perhaps save someone the headache!
GKMatchRequest *matchRequest = [[GKMatchRequest alloc] init];
matchRequest.minPlayers = 2;
matchRequest.maxPlayers = 2;
GKMatchmakerViewController *controller = [[GKMatchmakerViewController alloc] initWithMatchRequest:matchRequest];
controller.delegate = self;