Q: How to find someone's Myspace by their IP address?

A: Not possible unless you had access to myspace's server logs.

A: The only thing you can do with the IP address is know what ISP they use to access the Internet. If you had access to the MySpace server logs (and I doubt you do) and the software to read it easily, you could see what pages/accounts they visited. But that doesn't mean that they would be on their MySpace page, they could be looking at someone else's. You might try using the search field on MySpace if you know their screen name or something else about the person you're trying to locate there, like their real name, state they live in or some topic that they might have written about on their page. It will take a while but you might get lucky.