To add to what has been said, take note that if you are in your panel, over to the left you see the word Domains... click on it and some submenus open... you will see
- Manage Domains
Any domains that you have either purchased through or transfered to Dreamhost will show up under Registrations, but they must all (be they registered at DH or not) be added under Manage Domains to be hosted here. Note that there is a definite difference between registered and hosted... registered just means you OWN the domain name, but hosted means it is set up to publish content to the web on. You may add any domains that you own (no matter WHERE they are registered) to the Manage Domains hosting setup section.