What about this?
var products = new TreeViewItem {Header = "Products"};
var mediaPlayers = new TreeViewItem() {Header = "Media Players"};
var charts = new TreeViewItem() { Header = "Charts" };
var games = new TreeViewItem() { Header = "Games" };
products.Items.Add(mediaPlayers);
mediaPlayers.Items.Add(games);
games.Items.Add(charts);
MyTreeView1.Items.Add(products);