What is the "error page? A "not found" means the DNS has not propagated yet. You will be "very lucky" if the subdomain is reachable via http in only 30 minutes - DNS can take "up to" several days to fully propagate. Generally a subdomain for a previously listed domain will propagate much faster, but 30 minutes is really optimistic. Be patient and wait a bit before assuming it isn't working
As for your second question, the "whine" turns me off - check the wiki or google for the use of cron. You use it on Dreamhost the way you use it on any other nix box , and you do it through the shell.
Because they have given you , instead, a much more useful panel, as well as shell access, which combine to give you degrees of flexibility and power that are "orders of magnitude" higher than afforded by cPanel.
If you take the time to learn and become accustomed to the Dreamhost way, you will never be satisfied with the limitations of cPanel again .
Edit: BTW, it helps others more easily find answers in the forum if you break two seperate questions into two seperate posts - this really should have been two threads.