If you currently have the domain hosted elsewhere and you wish to move the hosting (but not the domain registration) to DreamHost, follow the procedure below;
During sign-up select the "I already own a domain, I just want to host it with you (you can still register a free domain later)!" option.
Enter the full name of your domain in the "First domain to host:" field.
Once your application has been approved and everything is functional, you can follow the instructions in this wiki article to create a free dreamhosters.com sub-domain to mirror your real domain. This will allow you to install and configure things before pointing the DNS to dreamhost and making the new site 'live'.
Sub-domains are not implemented as sub-directories here at DreamHost. Each sub-domain has its own web accessable directory and is basically treated the same as a full domain.
This means you will have to add your sub-domains via the panel at Domains -> Manage Domains -> Add Domain / Sub-Domain and create a new dreamhosters.com sub-domain to mirror each of them.
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