WriteLarge.com

Dotster may have cheap DNS registrations, but FYI: You can't change the A-Record on a Dotster registered domain without buying an extra service. Meaning that Dotster and Tumblr do not get along.
According to Friendly Doster Service Rep Jared H:

Unfortunately the only means we have for assigning an IP for a domain is to make A records in DNS Management, make records in a hosting package, or register new name servers.

I suppose a refund is out of the question.