Did you ever get the LeaderBoard going in your app? I followed Kaiji's advice and it was that simple. Setting up the Leaderboard in gamecenter wasn't that difficult, then I just used GameCenterInit(OnGameCenter) in AppMain, GameCenterLeaderboardPostScore() in my gameover function, then made a button that called the GameCenterLeaderBoardShow() command. I also checked for a flag in the OnGameCenter callback and if a flag was set I called the command AlertShow("Game Center Error!", "An error has ocurred. Be sure that you are logged or you have internet connection.") . I also setup seperate Leaderboards for when you play the game in easy mode or hard mode. One thing I should note is that when testing in Sandbox, your score will show up right away (as Me) but other account scores took hours before they showed up.
EricB