pass the model type to the action via a hidden field?

I have a question about the acts_as_commentable plugin. In the
instructions it says to use hidden fields to pass the model type and
model id. This is somewhat confusing me. I am trying to add comments
to a user object and am not sure how to specify the model type. This
is the link that I am using to get the comment view where one will
enter his/her comments:
<%= link_to "add a comment", :action => "add", :controller =>
"comments", :value => @user %>

Do I pass an instance of the user object through the value=>@user
above?
Here is my comments add page:
  <% form_for( :comment, :url => comments_path ) do |f | -%>
    <%= f.text_field :title %>
    <%= f.text_field :comment %>
    <%= f.hidden_field 'commentable', 'commentable', :value => @user %>
      <%= f.hidden_field 'commentable', 'commentable_id', :value =>
@user.id %>
    <%= submit_tag 'Create' %>
  <% end -%>

I know this is not correct, but I am very confused as to what the
@model.type and @model.id should each be. If anyone can give me a
lead as to the right direction it will be much appreciated! :slight_smile: thanks