The failure may be due just to the creation not yet having been processed. It can take up to five hours. You'll know, when you receive a "DreamHost Mail Account Activated" message.
And yes as Torben says, webmail login goes by alias but be sure NOT to use mailbox name - here I found this logs me into someone else's mailbox! And the webmail display doesn't say which it is logged into, leaving you to e.g. delete messages without realising you are doing damage.