Well, first of all you'll need to put some images in the tree view's image list. In order to prevent Plus! from messing up the image indexes, I always prefix the image ID with the image position.
<Control xsi:type="TreeViewControl" Id="MyTree">
<Position Left="10" Top="10" Width="100" Height="150" />
Now to add an item, you can use TreeView::addItem.
var tree = new TreeView(PlusWnd, "MyTree");
var parentNode = tree.addItem("My folder", 0, 0);
var childNode = tree.addItem("My file", parentNode, 1);
This will add a root node with a child node to the tree. The root node will have a folder icon and the child node will have a file icon (assuming that you have those images in your script).