Go here and you can see some client script that accesses fields.

http://www4.ewebcity.com/baroberts/

This site is no longer being updated(sorry). However, the client-side script examples may be useful.

Hidden fields are no different from any other in code. An example to create one is:
<input type=hidden name=hiddenfield value="xxxx"></input>

In server-side script you reference them the same as any other field that is "posted".(strHiddenField=Request("hiddenfield"))