If you want ALL of the domain that is Registered elsewhere to be HOSTED at dreamhost then follow the instructions in the post above.
However if you are trying to split the domain up, so that some services are here and some are there, then the process is more complicated and you should post back with more details about what you are trying to do, so we can provide a better answer for your situation.
FYI... After you change the nameservers as directed above, OR make any other change that effects DNS there is a delay before that change takes effect. This is called DNS propagation. Propagation can take anywhere from a few minutes to 3 days, with the avg delay being about 4-8 hours for simple DNS changes, however by nature nameserver changes probably run longer than the average.