You need to download and install this extension: centered.zip.

Before you install this you will first need to install the Macromedia Extension manager available free from macromedia.com

Follow the instructions contained in the extension and you are done.

Using this publish template has two added benefits: your movie doesn't scale (which is important if you have any bitmap images or alias text in your movie), and all of the stuff that is not on the stage gets hidden

That's it. Publish your movie. You might like to open the html file that gets produced when you publish your movie - just to see what it's doing or to change the background colour, etcetera.