I am not familiar with dreamhost but since you have a standard installation of roundcube then I guess you should have access to the roundcube files.
Using the file manager application from your panel,
go to your public_html directory (from here you can get the roundcube installation).
If roundcube is installed in a directory then it will be something like this : /public_html/<roundcube_directory/plugins/password>.
if not, then it will be /public_html/plugins/password
From either of those above as it applies to you,
locate the config.inc.php file.
View with an editor within the dreamhost panel (will recommend you download the file to your workstation to avoid you making changes to the file unknowingly).
Good luck