Hi guys -- purchased a copy of Machinarium years ago. I saw that the game has been updated since then, and the article I read says old owners could upgrade for free.
Hmm. I searched The Web for [ "machinarium definitive version" + free ] and the only article I found with those terms was
this one which states "Machinarium’s Definitive Version is now available as a free update on Steam, but only on Windows PC."
So that kind of presumes that someone wanting to upgrade must either have bought the game on Steam or somehow got a Steam key for the game.
Try writing to Amanita support, explain that you bought the game a long time ago, and then ask them for a Steam key.