This is simply a two-step process to complete. The first thing you need to do is go to your registrar and point your domain name to ns.rackspace.com and ns2.rackspace.com. They will then update your nameserver information and the changes will propagate over the internet. This change could take up to 72 hours to fully take affect over all the internet servers across the world.
The second thing you must do (if you did not do this already at signup of your MonsterCommerce store) is notify us of the domain name you will be using. There is a simple form at - http://www.monstercommerce.com/domain_change.asp. You can leave your old domain blank, and just enter your new domain, IP, and an email where we can reach you at to notify you of the change.