Machinarium (Mac OS X) saves its files in a cache, so if you accidentally clear your browsing history, all of your files disappear. Is there any way to back them up?
On Mac OS X you should find your savegame here:
To do a backup you can physically copy the file and store it away.
Anyway to get more informations take a look at this thread on Amanita Design.
Even better for you - the creator of the game has provided a tool to backup the saved game (great, for example, if you want to move from Steam to the direct download of the game without losing your save points).
The thread for it is here:
Simply download saves.exe and put it in the same folder as Machinarium.exe. Choose option 1 and you're sorted!