If you did NOT purchase your domain from us, we are not able to change the name servers because we don't control the domain. Please contact the place where you bought the domain and ask how to make the name server updates. Most domain registrars have a tool so you can do this yourself. Then you will remove the current name servers and replace them with the name servers from your welcome email.
We have flash movies on how to make the changes at a few of the more popular registrars. If your registrar isn't included you can still watch the movies to understand how it's done.
For more information, please read our DNS changes article.