Embarasingly easy. Crack it. Nothing more.
Ok- I just bought it. And subsequently solved it. There are four levels, culminating with "impossible". Wanna know the problem (besides being stupidly easy)? There's no way I've seen to pick the level if you want to quickly show a friend, say, easy level. It looks like there is, but sadly, there is not. You know why I bought this? Because the developer's description tantalized me with the promise of "more". That is, after I cracked the safe, I though (mistakenly, it turns out) there may be something else- something more to the game.
If you're bored, this is a good way to waste 15 minutes. No instructions? IT'S A SAFE. What else can you do, but open it? I think we've all seen the movies on how it's done, right? Do it.