How do you tell it's not working? Since the cron job doesn't redirect standard output to a file, it will be sent to the user's mailbox by default. Have you checked it?
If the user did not received any mail, first you have to establish if the cron daemon is running (as root)
# /etc/init.d/crond status
Then check the cron logs to see if the job did actually run or not. You should see entries like this in /var/log/cron:
Nov 24 09:00:01 host CROND: (user) CMD (echo "Hello World")
Nov 24 09:01:01 host CROND: (user) CMD (echo "Hello World")