<%= fields_for "form[fields][#{index}]", field do |f| %>
    <tr>
      <td><%= f.hidden_field :_type %><%= f.label :name, 'Name *' %><%= f.text_field :name %></td>
      <td>Multi-Select box</td>
      <td><%= f.label :label, 'Label' %><%= f.text_field :label %></td>
      <td><%= f.label :default, 'Default Option' %><%= f.text_field :default %></td>
      <td class="center">-</td>
      <td><%= f.label :options, 'Options' %><%= f.text_field :options %></td>
      <td><%= f.label :required, 'Required?' %><%= f.check_box :required %></td>
      <td width="25" class="up"><a class="page_up" title="Move this field up">up</a></td>
      <td width="30" class="down"><a class="page_down" title="Move this field down">down</a></td>
      <td><%= link_to_function 'Delete', 'return delete_form_field(this);', :class => 'delete' %></td>
    </tr>
<% end %>

<%= fields_for "form[fields][#{index}]", field do |f| %>
    <tr>
      <td><%= f.hidden_field :_type %><%= f.label :name, 'Name *' %><%= f.text_field :name %></td>
      <td>Multi-Select box</td>
      <td><%= f.label :label, 'Label' %><%= f.text_field :label %></td>
      <td class="center">-</td>
      <td class="center">-</td>
      <td><%= f.label :options, 'Options' %><%= f.text_field :options %></td>
      <td><%= f.label :required, 'Required?' %><%= f.check_box :required %></td>
      <td width="25" class="up"><a class="page_up" title="Move this field up">up</a></td>
      <td width="30" class="down"><a class="page_down" title="Move this field down">down</a></td>
      <td><%= link_to_function 'Delete', 'return delete_form_field(this);', :class => 'delete' %></td>
    </tr>
<% end %>

Remove ability to set a default for Multi-selects
Remove ability to set a default for Multi-selects

Doesn't make sense to have a default for this type of field
