SPSecurity.RunWithElevatedPrivileges(delegate()
{
 using (SPSite oSiteCollection = new SPSite("http://mySiteUrl"))
 {
       using (SPWeb oWeb = oSiteCollection.OpenWeb())
       {
             oWeb.AllowUnsafeUpdates = true;
             // Create the node.
             SPNavigationNodeCollection _quickLaunchNav = oWeb.Navigation.QuickLaunch;

             SPNavigationNode _SPNode = new SPNavigationNode("Link Title", "http://link_url", true); //i tried with false, too. It didn't work

             _quickLaunchNav.AddAsLast(_SPNode);
             oWeb.Update(); //added this since I've seen it in some examples, but not every example had it
       }
 }
});

public override void FeatureActivated(SPFeatureReceiverProperties properties)
{
    using (SPSite site = (SPSite)properties.Feature.Parent)
    {
        using (SPWeb web = site.RootWeb) {
        {
            // Add links to navigation
            SPNavigationNode listNode = new SPNavigationNode("Visitors", "Lists/Visitors/All Visitors.aspx");
            SPNavigationNode newVisitorNode = new SPNavigationNode("Register New Visitor", "Lists/Visitors/NewForm.aspx");
            SPNavigationNode siteContentsNode = new SPNavigationNode("Site Contents", "_layouts/15/viewlsts.aspx");

            web.Navigation.QuickLaunch.AddAsLast(listNode);
            web.Navigation.QuickLaunch.AddAsLast(newVisitorNode);
            web.Navigation.QuickLaunch.AddAsLast(siteContentsNode);

            web.Update();
        }
    }
}

// references
using Microsoft.SharePoint.Publishing;
using Microsoft.SharePoint.Navigation;
using Microsoft.SharePoint.Publishing.Navigation;

// code to add Navigation link
using (SPSite spSite = SPContext.Current.Site)
    {
        using (SPWeb web = spSite.OpenWeb())
            {
                        string headingTitle = "myPageTitle";
                        string headingUrl = "http://mysitename.com/SitePage/myPage.aspx";

                        // Get the Quick Launch headings.
                        SPNavigationNodeCollection ql = web.Navigation.QuickLaunch;

                        // If a Resources heading exists, get it.
                        SPNavigationNode heading = ql.Cast<SPNavigationNode>().FirstOrDefault(n => n.Title == headingTitle);

                        // If the Resources heading does not exist, create it.
                        if (heading == null)
                        {
                            heading = new SPNavigationNode(headingTitle, headingUrl);
                            heading = ql.AddAsLast(heading);
                        }
            }
     }

SPSite spSite = (SPSite)properties.Feature.Parent
using (SPWeb web = spSite.OpenWeb())
{
    SPNavigationNode listNode = new SPNavigationNode("Visitors", "Lists/Visitors/All Visitors.aspx");
    SPNavigationNode newVisitorNode = new SPNavigationNode("Register New Visitor", "Lists/Visitors/NewForm.aspx");
    SPNavigationNode siteContentsNode = new SPNavigationNode("Site Contents", "_layouts/15/viewlsts.aspx");
    SPNavigationNodeCollection nodes = web.Navigation.QuickLaunch;
    nodes.AddAsLast(listNode);
    nodes.AddAsLast(newVisitorNode);
    nodes.AddAsLast(siteContentsNode);
    web.Update();
}