Add custom validation to checkbox

Hi all,

My query is pretty simple. All I want to do is to have a checkbox that similar to "I have read and agree to the terms and conditions" checkbox and users have to tick the box to submit the form.

I followed this post http://nintexdownload.com/SL/Supportfiles/Custom_Validation_tutorial.pdf to add custom validation on the form using JavaScript. But the function was not triggered when I submit the form.

Help please. I'm using NF2010 1.2.1 

Many thanks,

June