Ah, yes. If the domain has been previously hosted under your first account, you will have to contact support to have them move it to your second account.
Dreamhost had to institute a policy within the automated system forbidding the hosting of domains that had been previously hosted here for a year or so. This is to stop people who had been "rolling" their domains from one promo-coded account to another to try to game the system and get $22.40 a year hosting forever.
Actually, now that I think of it, I'm not sure whether they put in the ability to add hosting for previously hosted domains on non-promo accounts. Hmm.
Finally, are you sure you've removed hosting of that domain from your first account? You certainly can't host the same domain in two accounts at once.
Extra lifetime domain and $82 off with code [color=#CC0000]1DOM82[/color] or use [color=#CC0000]LM97[/color] for $97 off. Other Dreamhost coupon codes