Arcade help was never completed so far. Arcade is still in "Beta" stage to be fair.
You can upload a game and in its properties ("Edit" button in the game's page), uncheck "Available for play".
The game will be available for you as "Removed game" which is pretty much the same as "not live".
You will have access to it in your game's list in the game manager, but it will not be listed publicly.
As seen in the How do I FAQ :
Make a leaderboard in the Scirra Arcade - LINK 1 LINK 2
So yes, you need your game uploaded in the arcade for the Scirra Arcade leaderboard to work.