Mark,
In reply to:
I had an email from Dreamhost, which said:
"Username: xxxxx
Password: abcabcabc (changeable through the web panel)
FTP Server: charybdis.dreamhost.com"
That, from your initial email, *is* your FTP username/password (which I guess you have now figured out). Once your domain name has DNS updated across the net, you can also log into "yourdomain.tld" as well as "charybdis.dreamhost.com" If you have no reason to want to keep your domain name private, posting it here would make it easier for some of us to help you, as we can check DNS status and possibly browse the site when DNS is updated to your DreamHost server (which is helpful for sorting out directory related issues sometimes), but that is up to you.
In reply to:
But I cannot see where to change the password even.
You can change the password for a given user from the Control Panel -> Users -> Manage Users screen - just click the "Edit" link under the "Actions column of the row for your username. Complete the form that appears entering your old and new password, submit the form, wait for it to "take", and you will have changed you
In reply to:
Also, then I look at the Manage Domains screen, it says: "FTP user / CGI-runs-as-user yyyyyyy"
Ah! *that* may be a clue to your WordPress "one-click" problem. is the "yyyyyyy" on that screen the same as the "xxxxx" in your "Welcome" email?
If you have more than one user involved here, you are likely having a "one-click" problem trying to add a WordPress site as a different user than the one that "owns" the 'yourdomain.tld" directory (which would also explain why you didn't see that directory when you logged in via FTP). Each directory on DreamHost is "owned" by one, and only one, "machine user"; if you are trying to install WP into the user space of a user other than the one that owns your domain's directory, you *will* have problems (likely including the ones you're describing).
--rlparker