The initial problem was that DNS hadn’t yet propagated which is a very normal thing, and just waiting would have solved the problem.
I wish dreamhost would include in their instructions to wait until you can see the dreamhost “coming soon” page on your domain or sub-domain before running the one-click installer.
To solve the problem at this point, you just need to log in via ftp and delete everything in the directory named the same as your domain name. Then run the one click installer again. Somehow you have confused the system about what’s installed. Use a real FTP client, don’t try to do this with webftp (webftp you have to do one file at a time and you may be at it for hours).
Remember this… any changes the affect DNS (adding or moving a domain or sub-domain) will result in a delay before those changes take effect. This is called DNS propagation and it literately can take anywhere from a few minutes to 72 hours(3days)… that said most of the time 4-8 hours is the average). Not everyone gets the DNS update at the same time either. You can use the tool at https://www.whatsmydns.net/ to track progress. The locations on the map are key servers and only give you a general clue about how quickly propagation is occurring.