sorry for being confusing. Let me first back track a little and ask, if you had a forwarding only E-mail address to send things to yahoo, have you changed things so that it is now not forwarding, but storing the E-mails at dreamhost for your to check?
Next, in the panel > Mail > Manage area you should see some information like this for your mailbox:
"Your Name" m6286549
1 MB / 200 MB (1%)
where this example shows "m6286549" it might show a user name instead. Whatever it shows there will be the user name you need to use in outlook. If you don't see either of those, you heavn't set up a mailbox - or it's still a forwarding only one.
Once we get that part figured out, see if this Wiki Article shows you anything new in the outlook setup (expecially the My server requires authentication part.
hopefully this post is more helpful.