What's your current hosting for the domain and the subdomain?
One easy way of doing this if you're starting from scratch is to:
1. Add the domain in the "Domains>Manage Domains>Add New Domain / Sub-Domain" page of the control panel. Parking the domain is the least amount of data entry, btw.
2. Go back to the "Domains>Manage Domains" page. Delete web hosting for the domain.
3. From the the "Domains>Manage Domains" page, click on the "DNS" link for the domain. Add a blank "A" record and a "www" "A" record with a value equal to the IP address of your server at your other host.
4. From the "Domains>Manage Domains" page, click "Add New Domain / Sub-Domain" again. This time, add the subdomain as a fully hosted domain.
You should be all set up after DNS propagates. The main domain should point to the other host and your subdomain will be hosted here at DreamHost.
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