Hi Jean,
Thanks for jumping in. I think the main issue for those of us trying to work with Steamworks.net and Playmaker is that the Steamworks functionality in the actions as they stand right now is incomplete.
Some things don't seem to work as intended. It's difficult to know, though, because the actions don't seem to report errors or allow Fail results. (As mentioned, its support is "incomplete".)
As a case in point, while I have been able to get a leaderboard to update, it only does so occasionally, and without displaying any reason for failing when it doesn't work.
Also, there doesn't seem to be any actions available for grabbing the information to display in-game. Seems to be a one-way trip for leaderboard data, for example.
My understanding is that the Steamworks.net package may not be official Playmaker actions, but I'm hoping you guys might be able to take a look and shed some light on where Playmaker is at with Steamworks support, and perhaps give it some much-needed love.

Looking forward to your response.
Regards,
Mark