Just to verify that you are following the correct procedure...
Goto Domains -> Manage Domains
Click on Add New Domain / Sub-Domain
Enter your domain name in the Domain to host: section.
Confirm that FTP user / CGI-runs-as user: is set to a valid user.
Click Fully host this domain now!
Your domain should then be visible in the Manage Domains section of the panel (it may take a minute or two) and you should have a new sub-directory in your home directory with the same name as your domain.
If that doesn't work for you, I suggest lodging a support request through the panel, something might be broken.
Save [color=#CC0000][b]$50[/b][/color] on DreamHost hosting using promo code [color=#CC0000][b]SAVEMONEY[/b][/color] ( Click for promo code details )