It is relatively simple.
First thing you need to do is to add a leader board to your app on iTunes Connect and give it a Leaderboard ID there.
Then, in C2 - first "Request Login" (I did this on start of the layout) and then on your hiscore-event "Submit score" using the Leaderboard ID that you gave on iTunes Connect.
There you go!
Oh and you should know there is a CocoonJS bug on the "Show Leaderboard" action - it opens the leaderboards nicely but when you get back to your game the touch controls are stuck. I reported and documented this to Ludei already, so hopefully they will fix it soon.