How do the batteries go in the compartment? I.e. facing up, down, a combination? I've tried a couple of different ways and it doesn't work. There's no diagram on the package and I don't see a plus or minus sign in the battery compartment that would clue me in on how to install them. Plus the lid of the compartment is very hard to slide in and out, so I'm afraid I'll break it before I even get to use it! Getting frustrated here, help!