Simply go to Domains -> Registrations, tick the relevant domains under the Modify Whois? column then click the Modify whois for selected button.
Now make sure all four tick-boxes at the top are selected (Owner, Admin, Tech and Billing), make sure Use DreamHost's free WHOIS privacy service.. is selected then click the Update contact info for this domain! button. Thats all there is to it.
Note, it can take a little while for the changes to take place.
Save [color=#CC0000][b]$50[/b][/color] on DreamHost hosting using promo code [color=#CC0000][b]SAVEMONEY[/b][/color] ( Click for promo code details )