Leave DNS management at the old host too since email requires several specialty DNS records.
On dreamhost add the domain name as fully hosted to the Manage Domains page of the panel. Give it 20 minutes to complete setup. Go back to manage domains in the panel and click the DNS button for the domain. After the page reloads the "dreamhost managed" entries are listed there. The very first one should be an A-record for the domain, the IP listed there is the one you need, write it down or copy and paste it.
Now log into your old host and use that IP address to create a record for the domain. Give it 4-8 hours and it should be working.