using DotNetBrowser;
using DotNetBrowser.DOM;
using DotNetBrowser.Events;

...

Browser browser = BrowserFactory.Create();
browser.FinishLoadingFrameEvent += delegate(object sender, FinishLoadingEventArgs e)
{
    if (e.IsMainFrame)
    {
        DOMDocument document = e.Browser.GetDocument();
        DOMSelectElement select = (DOMSelectElement) document.GetElementById("select-tag");
        SelectOptionByIndex(select, 2);
    }
};
browser.LoadHTML("<html><body><select id='select-tag'>\n" +
        "  <option value=\"volvo\">Volvo</option>\n" +
        "  <option value=\"saab\">Saab</option>\n" +
        "  <option value=\"opel\">Opel</option>\n" +
        "  <option value=\"audi\">Audi</option>\n" +
        "</select></body></html>");

...

private static void SelectOptionByIndex(DOMSelectElement selectElement, int index)
{
    List<DOMOptionElement> options = selectElement.Options;
    DOMOptionElement option = options[index];
    option.Selected = true;
}