Click on "settings"...then "publishing"...then select publishing on a custom domain. You will also have to create a cname record in your backend of your domain host that points to ghs.google.com(Unless you buy domain through blogger). There are more detailed instructions on how to do this is you click on "setup directions", in the page in blogger where you select to publish on a custom domain.
does changing the CNAME of www do the job... and how long does it take for this effect to be seen.... Thanx a lot for the reply Rebecca
Don't they have a help guide? If you're using a domain with existing PR don't try use it to fool people into buying PR4 links etc.