Wait a second, i may be confused. If it's a URL at another site pointing to an existing dreamhost website (which is how I read the original post) then there are two option.
You can log in where ever the domain is registered and create a re-direct there to point to the dreamhost site,
Or second even better, redirect the namesservers as Ryo-ohki mentions, then create the re-direct on the dreamhost side, under manage domains.
Sorry if I have added unnecessary confusion.