First navigate to manage domains in the panel and find the domain in question. Look at the "web hosting" column. The "user" listed there is the one you should be logging into ftp with.
When you log into FTP you will be placed in that users home directory, where you should see a directory or folder named the same as the domain name. Change into that folder.
When you used the one click installer there was a drop-down for the domain name, to the right of that drop-down was a / followed by blank for a folder name, if you entered one you should now switch into that directory.
PS - are you sure your in the correct forum? Dreamhost doesn't use a www/ folder