The tumblr instructions are found here: http://www.tumblr.com/docs/en/custom_domains
for the dreamhost specific portion:
-If you have done anything so far as a trial, undo that first.
-Navigate to manage domains in the panel.
-Click the DNS link right under the fully hosted domain in question (the base domain to which you want to add the tumblr. sub-domain)
-After the next page loads, scroll down slightly and fill in form titled "Add a custom DNS record to mydomain.com"
Remember that DNS changes take time to propagate. For a new, never before set up sub-domain it should be pretty quick. On the other hand if you have been experimenting and use the same sub-domain name it may take longer (12-24 hours) as you wait for cached records to expire.