I've built and released a new game using Flowlab 'I Want Candy' that's now on the App Store.
I wanted to see the leaderboard in action as I need to use it on another project so I worked up an old demo.
Here's the link if you would like to check it out: https://itunes.apple.com/gb/app/i-want-candy/id1269287917?mt=8
Here's the link to the Flowlab version to see the leaderboard logic: http://flowlab.io/game/play/5880
There's a leaderboard button on the homepage and a leaderboard on the game over page that have a node of logic to display the leaderboard with an id. This id also matches the one in iTunes Connect you input when you set up the leaderboard there. Usually the id is: domain.gamename.hiscore. There's also a node connected to the high score logic, this only connects if the high score has been beaten. This updates the high score in the Apple Game Center.
The leaderboard seems to be working, if you notice any bugs please shout! I put a timer on the game so you have to grab that top candy!