Just check the tutorial section and search the forum, quite a few examples out there.
For the google and facebook leaderboards, you require some patience after setting up the leaderboard on the dev consoles.
It takes some time before they work.
Also, generally they require a working model from a secured hosting location (https) if you use web versions.
(So preview will most likely never work to send scores etc)
Make sure to check the log console (ctrl + shift + J) to see what errors pop up when your trying to get it to work.
They are often a good starting point when something is amiss with services from a 3rd party location.