The rails api for Builder::XmlMarkup uses both an ‘xm’ and ‘xml’ object to show how to work with xml, but only the ‘xml’ object seems to work. It also doesn’t clearly show how to setup the controller or that the code should go in an xml.builder file. Similarly here, the xml object is used in the view correctly, but also fails mention anything about the controller or type of file the code should go in.

Over at xml.com, the tutorial creates an @xml instance variable of type Builder::XmlMarkup.new and then uses that object in an .rxml view

xml.instruct!
xml.posts do
  @posts.each do |post|
    xml.post do
      xml.title post.title
      xml.body post.body
      xml.published_at post.published_at
      xml.comments do
        post.comments.each do |comment|
          xml.comment do
            xml.body comment.body
          end
        end
      end
    end
  end
end

You can also look for some updated documntation on this link http://developer.apple.com/tools/customizeonrailsleopard.html