Here is a method to add a node to the quick launch.  You need to pass in the name you want displayed for the link, the Site,  and the path to the page you want the link to open.
This method checks if the link already exists and it won’t add the node if it does.  So it’s safe to run multiple time without jacking up your quick launch.

private void AddNodeToQuickLaunch(string linkName, string SiteToOpen, string pathtoPage)
       {
           try
           {
               using (SPWeb web = SPContext.Current.Site.OpenWeb(SiteToOpen))
               {
                   web.AllowUnsafeUpdates = true;

                   SPNavigationNodeCollection nodes = web.Navigation.QuickLaunch;

                   SPNavigationNode createNode = new SPNavigationNode(linkName, pathtoPage, true);

                   foreach (SPNavigationNode node in nodes)
                   {
                       bool nodeExists = false;
                       if (node.Title.Equals("Lists"))
                       {
                           foreach (SPNavigationNode child in node.Children)
                           {
                               if (child.Title == linkName)
                               {
                                   nodeExists = true;
                               }
                           }

                           if (!nodeExists)
                           {
                               node.Children.Add(createNode, node);
                           }
                       }
                   }
                   web.AllowUnsafeUpdates = false;
               }
           }
           catch (Exception ex)
           {
               throw ex;
           }
       }