He wasn't talking about registration. He was talking about DNS resolution.
After you tell your registrar what nameservers you want to use (the ones I said to use before if you remember) then in about two days or less everyone on the Internet will be connecting to those DreamHost nameservers at some point to get new information.
In order to get your domain listed in those nameservers, you have to go to Domains -> Manage Domains and add it. Since you're hosting the site wioth DreamHost, you choose the "Fully-hosted" option. Your zone info gets setup and IP addresses are assigned by this process.
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