I didn't take a close look at your code -- looking at a bunch of Struts tags is less than illuminating -- but the premise of the dependent drowdown is fairly easy.
A cursory look at your code reveals that you are doing all the Ajax stuff directly. That's madness in my opinion. Better to get acquainted with one of the toolkits that vastly simplifies the process by masking browser dependencies and that handle the ready-change protocol. I'm partial to Prototype and jQuery myself.
If you want to implement your resource as a Struts Action, then you would address it just like any other action. The difference would be in what the Action returns as the response. Instead of a full HTML page, the response would be the text of a JSON construct or an XML document or however else you decide to return the option data to the client.