Normally, you would enter the promo code during step 5 of the sign up sequence ('Verify Total').
However, it sounds like you used a previously created Web-ID and added a plan using the panel. I am not sure you have the option of using a promo code under these circumstances.
Email DreamHost and explain the situation, I am sure they will work it out. You can use the 'Submit Support Request' option in the panel to do this (Support -> Contact Support -> Submit Support Request).
Save $50 on DreamHost hosting using promo code "SAVEFIFTY" ( DreamHost Promo Code )