When you sign up with DreamHost you have to enter your promo code in step 5 of the registration process.

In step 1 you select your plan and in step 2 your domain or subdomain. In step 3 you either select your web id or choose to create a new one and in step 4 you enter your personal info.

Then in step 5 ("Verify Total") you have the option to enter an Optional Promo Code. Enter your selected promo code and click update. Your discounted price should now be shown. Saving money!