Unfortunately I dont have the time to do it for you as I have many projects on the go right now.
In a nutshell....
The extension has two different parts. A server behavior which populates the data in an array when the page loads. It sounds like you have figured this part.
For the second part you need to select the trigger for the array update. This will be the parent drop-down (i.e the one with the parent values that will be passed to the child). This action is client side so you select the dropdown form element and then go to the TAG INSPECTOR window in Dreamweaver, click behaviors and then select the populate list from array option. In here you set the source and target.
However, before continuing I suggest you get to grips with the different between server side and client side actions before you go any further. It's much easier to deal with this (and support your web site in the future) if you know how it works.