From looking at this thread, it occurs to me that your problems might have been due to the fact the Control Panel has been "acting up". Dreamhoststatus.com has reported various "up"/"down"/"broken" issues with the panel since around 10:00 PM PDT (GMT-8) on the 13th, and those problems continued to a greater or lesseer degree until the evening of the 16th.
The main difference, is that a "mailbox" is an actual email account on the Dreamhost system that provides POP3/IMAP functionality, disk storage for mail, etc., while an "address" is just an "address" that can be associated with a "mailbox" or can be handled in other ways, such as forwarding, bouncing, etc.
Whever a mailbox is created, it has an address asigned to it, but an address can be created without having an associated mailbox.
Now that the panel has settled down, I'd suggest trying again, very carefully reading each screen, and see if you still have problems.