Hey,
I made a file using VI, called ez-ip:
#!/bin/bash
# Script to update the IP-adress for dyndns.org
ez-ipupdate -S dyndns -u ***ACCOUNT***:***PASSWORD*** -h ***ACCOUNT***.dyndns.org -i eth1
Replace ***ACCOUNT*** with your account name, and ***PASSWORD*** with your password, save that file in your etc/cron.weekly folder, and it works fine for me

Good luck!