Quite a few people ask this question when first starting out with DreamHost. I guess this is due to them coming from other hosts, using cPanel etc, where things are done a little differently.
For the benefit of others in the future; Adding sub-domains at DreamHost is the same as adding full domains, you add them at Domains -> Manage Domains -> Add New Domain / Sub-Domain. Simply enter the full sub-domain in the Domain to host: field. Of-course, the parent domain must have already been added to the system.
Save [color=#CC0000]$50[/color] on DreamHost plans using [color=#CC0000]PRICESLASH[/color] promo code (Click for DreamHost promo code details)