Getting the page inside <frame> element or <iframe> element can be done by using HtmlPage.getFrames().
Suppose you have the following page: 

<html>
  <body>
    <iframe src="two.html">
  </body>
</html>

You can use the following code to get the content of two.html : 

        final List<FrameWindow> window = page.getFrames();
        final HtmlPage pageTwo = (HtmlPage) window.get(0).getEnclosedPage();

Another example that navigates API docs to get a desired page of a class: 

        final WebClient client = new WebClient();
        final HtmlPage mainPage = (HtmlPage) client.getPage("http://htmlunit.sourceforge.net/apidocs/index.html");

To get the page of the first frame (at upper left) and click the sixth link: 

        final HtmlPage packageListPage = (HtmlPage) mainPage.getFrames().get(0).getEnclosedPage();
        packageListPage.getAnchors().get(5).click();

To get the page of the frame named 'packageFrame' (at lower left) and click the second link: 

        final HtmlPage pakcagePage = (HtmlPage) mainPage.getFrameByName("packageFrame").getEnclosedPage();
        pakcagePage.getAnchors().get(1).click();

To get the page of the frame named 'classFrame' (at right): 

        final HtmlPage classPage = (HtmlPage) mainPage.getFrameByName("classFrame").getEnclosedPage();