If your not finding the wordpress files the one click installer installed for you, then it's an error on your part. The one click installer does install the wordpress files to your account.
Start at the "manage domains" page of the dreamhost panel. Find the entry for the domain or subdomain in question and look at the web hosting column". It will say "fully hosted" and have a user name. This is the user name you MUST use when logging into FTP.
Once you have logged into that user, there will be a directory that was created with the same name as the domain/sub-domain name, switch into it. Now did you install WP at the root of your domain/sub-domain or put it in a folder such as /wp /wordpress /blog etc? if you did switch into that directory. You are now at the top level of your wordpress install, mostly you will only see other directories at this level, but those directories contain everything.
Also, if you delete your wordpress installation via this method, be sure to return to the one click installer page and remove the entry. The one click installer, by default, may be set for automatic updates.
A word about FTP: FTP is a very old (30 years, October 1985, RFC 959), very unsecure protocol. When using it your password is sent over the internet in simple plain text and is very interceptable. Switch to using SFTP. Also, in the dreamhost panel go to the "Manage Users" page and edit each your users and check the "disallow FTP" box. AFAIK, this does prohibit you from using dreamhost's webFTP (pydio), however it's far more efficient not to use that anyway since it has several critical limitations. See also http://wiki.dreamhost.com/SFTP and http://wiki.dreamhost.com/FTP#FTP_client_software