To add the links in the left navigation first we need to get all links in the quick launch bar using SPNavigationCollection class
SPWeb web = SPContext.Current.Web;
SPNavigationCollection coll = web.Navigation.QuickLaunch;
/* With SPNavigationNode class we are adding our custom links(Node) in the quick launch bar */
SPNavigationNode node = new SPNavigation(linkTitle,url,true);
/*Add the node as root in the quick launch bar*/
web.AllowUnSafeUpdates = true;
coll.AddAsFirst(node);
/*Give permissions to the link*/
node.Properties.Add("Audiance",";;;;","groupName or UserName");
node.Update();
/* Creating sub links under main links in quick launch bar */
SPNavigation subnode = new SPNavigation(linkSubTitle, subUrl,true);
node.Children.AddAsLast(subNode);
/* Providing permssions to sub links in quick launch */
subNode.Properties.Add("Audiance",";;;;","group or username");
subNode.Update();
web.AllowUnSafeUpdates = false;
thats all !!!!
