I am not a Mac user, but I believe iWeb is primarily aimed at making webpages destined for .Mac.
However, I believe you can tell iWeb to 'Publish to a folder', you can then upload the resulting files from that folder to the DreamHost web-directory for your domain. You can use the DreamHost supplied web-based FTP client to transfer the files. To access this FTP client, go to Domains -> Manage Domains in the panel, then click the WebFTP link next to your domain.
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