Rails 3: perform controller action upon selecting menu item (UJS?)

In Rails 3, I have a form that has a menu on it. I would like to render an area containing text fields within the form differently depending on what's selected in the menu.

Prior to Rails 3, one way it appears I could do this is by using observe_field on the menu and call an action in my controller that renders the appropriate partial which could then be swapped into a div in my form. In Rails 3, I'm not getting any further with this approach than the following error when I try to display the view the first time:

undefined method `observe_field'

Am I on the right track, or is there a new and improved, UJS-correct way of doing this in Rails 3?

Try this http://railsforum.com/viewtopic.php?id=37984