Assuming your domains nameservers are NOT pointing to dreamhost nameservers at this time.
Add yourdomainname.com on the manage domains page in the panel. Because your nameservers are pointed elsewhere no traffic will arrive at the dreamhost server.
To work on your domain you could add a dreamhosters.com sub-domain as a mirror of yourdomainname.com…
BUT… that was a great solution back before about 2002… today, not so much…
Today, especially if your going to use web app like Wordpress, Joomla, etc etc it really works much better to use the HOSTS file method to access your site at yourdomainname.com for the period of time your working in your site.
If you do choose to say use Wordpress AND the Dreamhosters.com subdomain to develop the site that’s fine but it’s a huge PITA when you make the site live to everyone. (it’s about 5 complicated steps that you just plain don’t have to do if you used the HOSTS file method.)
Alternatively, my favorite trick: I keep my router at home setup so that All I have to do is log into the router and click the radio button for “USE THESE DNS SERVERS” (the IP addresses of dreamhost nameservers are sticky they remain in the fields) and hit SAVE, then Reboot the router. When I’m done working on something at dreamhost, I log back into my router and click “USE MY ISPs DNS SERVERS”, Click save and reboot the router.