Google Maps API Issue

So I'm trying to make a little Google Map overlay, that will reload data from a remote server every time the page it contains loads. Obviously, I read the documentation, look at examples, sign up for a Google API key. But it's not working, and I'm getting error messages that the API key is "for a different website" and that I can go get another one. I now have three, which the docs say I shouldn't. Any ideas?