I have tried saving in both Safari and Firefox on Mac OS X, as well as with the standalone Mac download version, and saving has worked fine for me in all three. Just hit "save", go about your other business, and when you come back, hit "load." Simple as that.

Perhaps if you're still having saving difficulties, you should post more details so the developers can figure out what's happening. The only case of not loading I have seen was pure operator error - they didn't know to click "load" to restore the game.