If you have a domain name held with another domain registrar (such as Godaddy, register.com, network solutions etc...) you need to update your name servers so that they "point" to our servers.
Name servers are 2 simple strings that look something like this (yours may have a different number after the ns part, but they are included in your New Account Information email we sent you);
So when someone types your domain into their web browser, or sends you an email, the name servers tell the browser to go to your website stored on our server. Every domain registrar will let you update your name servers in their control panel or you can simply contact them and tell them to update them for you.
Once you update your name servers it will take a few hours for them to actually update online. So within 24 hours your domain will start pointing to our servers. While it is updating, your domain will still point to your old server so you won't see any of the changes you publish to our servers.
After 24 hours everything will start working as expected (it can actually take just a few hours but we give it up to 24 hours just in case).