All the one click installer does is install the current WP distribution as you would if you did it manually. It cuts out about 5 or 6 manual steps… Other than that…it’s the same thing.
That said to find your files:
Navigate to manage domains in the panel, find the fully hosted domain in question. Note the user name as listed in the web hosting column.
Log into FTP or SFTP using the USER from the step above.
The dreamhost default is to store that domains files in a directory of the same name under that user.