You should have got one when signing-up. Did you transfer a domain as part of the sign-up procedure? and did DreamHost extend that registration by 12 months as part of the transfer?
If the answer to those questions is no, then you should already have one unused domain credit. If you don't have a credit, I suggest contacting support they might be able to shed some light on the situation.
As for the other part of your question; You should receive another domain credit slightly before your plan anniversary date.
Save [color=#CC0000]$50[/color] on DreamHost hosting using promo code [color=#CC0000]SAVEMONEY[/color] ( Click for promo code details )