I would say either email or social media. If they have a Twitter account send them a message, if they don't respond after that (they should do) then send them an email.
No I meant send a message to the developer of the game you're wanting to play, not to the itch.io admin team. If they've uploaded their game to itch.io you'll probably find their contact details (Twitter, email, Facebook.etc) on their itch.io profile page or the game page.
Itch.io simply acts as a marketplace and doesn't have any involvement in the development of the game.
Thanks a lot for the help, I'm contacting the team.
Anyway, I'm really suprised about the reliability of Steam, differently from other good platforms...
Steam really isn't as good as what many people would lead you to believe - I personally have a few choice words to say about the Steam "service" (although my reasons for disliking Steam are probably different to yours.)
I do have a Steam acc, but I've not logged into it since mid-2012 after I saw how much of a wreck it really is.