For the domain that is registered elsewhere you will need to update the DNS settings of the domain to point to dreamhost by changing them to
Once this is changed with your domain provider you can treat the domain just the same as the one purchased from dreamhost.
For the domain to be used with tumblr http://www.tumblr.com/help/custom_domains shows that you need to set the A record in the DNS settings (click DNS under the domain name in manage domains). If you want to use something like subdomain.domain.com you would just create a new A record called subdomain and put the correct ip address (188.8.131.52) in as a value.
If however you want to change the A record for the actual domain to use domain.com with tumblr you will need to change the hosting settings to DNS only, this is at the bottom of the webhosting options. If you choose this you can then edit the A_record in the DNS settings.
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