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();
        DOMElement select = 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(DOMElement selectElement, int index)
{
    List<DOMNode> children = selectElement.Children;
    List<DOMElement> options = new List<DOMElement>();
    foreach (DOMNode child in children)
    {
        if (child is DOMElement)
        {
            options.Add((DOMElement)child);
        }
    }
    DOMElement option = options[index];
    option.SetAttribute("selected", "selected");
}