The crontab entry does not run every 10 minutes, just at 0 and 10 minutes of every hour. To make it run every 10 minutes, you can try
*/10 * * * * root /usr/sbin/ddclient
Now the silly questions: 1) is the cron daemon running? 2) how did you edited the crontab (crontab -e or whatever)? 3) did you receive any error message in the crontab owner's mail?
Edit: note that the user name (root) in a crontab entry is allowed only if this is the system crontab file (e.g. /etc/crontab).