Did you register this domain during the sign-up process (your 'free' domain), or did you register it later through the panel?
If I recall correctly, domains registered through the panel also need to be added to the DreamHost hosting system via Domains -> Manage Domains -> Add New Domain. You would do this after receiving the email telling you that the registration went through successfully.
If you registered the 'free' domain during the sign-up process, then this is automatically added to the hosting system.
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