It's more of a "feeling" than a physical place. A good tip from my sensei when I first started was to tense my tummy muscles - you will feel a bit that you can't tense and that is the spot you should visualise as your centre, or "one- point". As it happens, that is a point a few inches below the navel. I also tend to visualise my "one-point" within me, rather than on the surface of the skin but I don't think it matters that much - it's more the feeling of being co-ordinated that you are looking for.
At least that's how I do it