Hi unfortunately you cannot do this in a developer edition. Most likely due to licensing concerns.
"You can mask your default site URL with your custom domain name. Simply create a CNAME alias via your 3rd party domain name registrar and point your custom domain name to your Force.com domain name and populate the Custom Web Address field with your custom domain name. You cannot do this for Developer Edition environments or Developer Sandboxes. "
From http://wiki.developerforce.com/page/An_Introduction_to_Force.com_Sites
You should however be able to see the site using the standard domain which will be in a format like http:// MyDomainName-developer-edition.na6.force.com/ you can find yours at Setup - Develop - Sites