seaches with ajax......

I have problems using ajax in a search field.
In view/layouts I have main.html.erb with code:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
      "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="it" lang="it">
<head>
<title>Consegna sacchetti</title>
<%= stylesheet_link_tag 'scaffold' %>
<%= javascript_include_tag :defaults %>

..............

in application.controller I have put layout "main".

under view/sacchetti I have index.html.erb for sacchetti_controller:

<div class="searchEmployeeForm_adv">
<% form_tag'javascript:void(0)' do %>
   <p>
     <%= label_tag :search, 'Nominativo:' %>
     <%= text_field_tag :search, params[:search] %>
     <%= image_tag("indicator.gif", :id => "spinner", :style =>
"display: none;") %>
   </p>
<% end %>
<%= observe_field(:search,
                :frequency => 0.5,
                :update => :search_hits,
                :loading => "Element.show('spinner')",
                :complete => "Element.hide('spinner')",
                :url => {:action => :list, :method => :get},
                :with => "'search=' + escape(value)")%>
</div>
<div id="search_hits">
<%= render :partial=>'rusers_list' %>
</div>

observe_files seems don't work, with firebug I see no ajax requests.
What I've missed?

p.s. sorry for my bad english