The best way to set up a seemless transfer is if you already have an existing fully hosted domain with dreamhost.
That is, one that is already set up & viewable from the web.
Seamless transfer steps:
1) Add a new domain to your account
2) create mailboxes and addresses for your domain (so you don't miss your emails once you transfer)
3) to connect via FTP like this:
host: existingdomain.com <-- a working "fully hosted" domain on your DH plan
directory: nextsite.com <-- the name of the new website
4) When you are ready point your DNS to DH, your site will be ready to go.
It's a crazy workaround… but it works.
Hope that helps…