First, check your username and password associated to the subdomain. You can check via DH panel --> Domains --> Manage Domains --> Web Hosting --> Edit. Make sure that you are using the right one.
Second, the setting of the Dreamweaver should be:
ftp name: domainname.com
Did you miss anything?
$50 off and 3 free domains with code: [color=#CC0000]DH3[/color] [url=http://dreamhost.com/r.cgi?296804/signup|DH3]Sign Up NOW[/url] or More Codes Here