If you want to do any useful work with your new domain, you will need to either:
a. Change the DNS to point to the new nameservers, or
b. Create a subdomain (such as yoursite.dreamhosters.com) on the dreamhosters.com domain.
If you go the (b) route, you can then create a MySQL hostname with that, eg. mysql.yoursite.dreamhosters.com, and setup your database. You access phpMyAdmin by browsing to the hostname and entering your username and password.
Once everything is ready, you can change your DNS to point to the new site, create a MySQL hostname from your own domain, and then alter your code to reflect the change of hostname.
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