I would have guessed that Lensman was right about it not being added in the hosting section... but you seem surprised that your site doesn't show up. Did you upload your site to Dreamhost?
If all you did was transfer the domain name, you still have work to do. See what happens if you try to add the domain... if it says it's already in the system, but you don't see it, then you'll probably want to contact support.
If you can add it, then it wasn't already there. Then you'll need to upload your files as the user it's assigned to, into the directory that's named after your domain.
Also, if you transferred the domain here (it will show up under registrations), and you need time to get things worked out (big site?), you could temporarily point the nameservers back to your old host--if you didn't cancel the account there.
Then, get everything squared away here and switch them back.
If you do that, this wiki article walks you through making sure your site is up and running here, before changing the nameservers back to ns1.dreamhost.com, ns2.dreamhost.com and ns3.dreamhost.com.
Also, don't forget to go into the mail section and create your email addresses. That stuff isn't automatically carried over from the last host. If you're using an offsite mail service like Tuffmail, Fastmail, etc... then this isn't anything to worry about.
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