The plugin is bundled in r207. It is in the category "Platform specific".
There is no manual article yet as it is only updated on stable updates.
Though it is quite simple, the plugin only has a single action "Submit score" and takes two fields.
The "Leaderboard ID" (name likely to change in the long run) will be the name of the leaderboard you've created in the games manager page on the arcade itself. The other field is the score you want to submit.
From the previous link click on "Your games" in the left column.
Then you have a list that contains your uploaded games on the arcade. On the right of each game there is a "Leaderboards" link, click it.
It opens a specific page that allows you to create several leaderboards for each games as well as a list of the existing games.
That should be pretty straightforward from there.
Consider the arcade and release are still beta, so subject to change in the future before the next stable release.