Sorry if it wasn't obvious, but I was replying to Nathan's post, not yours.
Unfortunately, that appears to be the case.
However, you still have the option of contacting DreamHost via the support section of the panel and telling them you meant to use a promo-code, they may (or may not) be sympathetic.
If you do decide to go this route, I would suggest trying the 9999 promo-code, which I believe is active this week-end, this code is DreamHost's own anniversary code and provides a $99.99 discount.
Save [color=#CC0000]$50[/color] on DreamHost plans using [color=#CC0000]PRICESLASH[/color] promo code (Click for DreamHost promo code details)