I tried a simple example, but the SiteNavigationElement is not working when I test it using the Google Structured Data Testing Tool. It gives the error:
SiteNavigationElementis not a known valid target type for theadditionalTypeproperty.
The Microdata:
<div itemscope itemtype="http://schema.org/WebPageElement">
  <link itemprop="additionalType" href="http://schema.org/ItemList" />
  <meta itemprop="name" content="navigation_menu" />
  <ul>
    <li itemprop="additionalType" itemscope itemtype="http://www.schema.org/SiteNavigationElement">
      <span itemprop="itemListElement">
        <a href="http://www.example.com/link_1" itemprop="url">
          <span itemprop="name">Link 1</span>
        </a>
      </span>
    </li>
    <li itemprop="additionalType" itemscope itemtype="http://www.schema.org/SiteNavigationElement">
      <span itemprop="itemListElement">
        <a href="http://www.example.com/link_2" itemprop="url">
          <span itemprop="name">Link 2</span>
        </a>
      </span>
    </li>
  </ul>
</div>