Are you editing the cron file manually, as opposed to letting the dreamhost panel maintain the entry? That's fine if you are, just being clear. When you type crontab -l at the shell prompt are you seeing your entry?
If that's working the next thing I would do is drop the redirection on the end of the command for troubleshooting purposes... I.e remove ">/dev/null 2>&1" from the end. When you do this make sure you have a valid email listed in the MAILTO line at the top of the file. With the redirection removed you should get an email from cron every 5 minutes. Let us know if you need help interpreting that.
FYI - the best email address to use is one hosted at dreamhost, to make certain these emails arrive. They may or may not to another type of address (why is a whole topic by itself).