If you have been sent the username/passwords by e-mail, then it depends.
You can login to FTP to upload files soon after by logging into [machine].dreamhost.com, where [machine] is the “Server” handling the Web service for your domain. After the DNS change has propagated, you’ll be able to use your ftp.[yourdomain] or just [yourdomain] instead. You can lookup the “Server” in the Domains -> Manage panel.
For webmail, obviously the DNS change has to have propagated to use webmail.[yourdomain] but you might be able to use webmail.dreamhost.com instead while you wait.
Usually it takes less than 2 hours for the username/passwords to be setup
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