When you said you were able to mirror the site what did you mirror it to? If it is your original site then that's not quite right, but I'm lead to believe by this follow-up question that may be the case.
In short, what should have happaned so far:
1 - add your domain in the dreamhost panel as fully hosted. You will either add it to an existing ftp user, or create a new ftp user during this process. If you can go to 'manage domains' in the panel and see your domain listed as 'fully hosted' then this step is complete. The ftp user that you should use will be listed there.
2- mirror your domain. Since accessing your domain from outside of Dreamhost will point you to your old site, you create a mirror that points to the dreamhost site that can only be accessed at this point via dreamhost's DNS.
If you did a one-click install and then mirrored that you are in great shape. Then look up your SERVERNAME via the "account status" link in the panel. Next FTP/SFTP to SEVERNAME.dreamhost.com and log in with the username found on manage domains.
After logging in you will be in your home directory, you should see a directory listed within named the same as your domain name, that is where your WP files will be found (with perhaps another directory in the chain depending how you installed it).