When you are populating the treeview create the tag when you add the node...
Dim tn As TreeNode
tn = treeView.Nodes.Add(Me.Nodes(i).ToTreeNode)
tn.Tag = i
Then read the tag on an appropriate event (e.g. click or afterselect etc)
Me.TextBox1.Text = (e.Node.Tag)
or update it
Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
If Me.TreeView1.SelectedNode Is Nothing Then
Exit Sub
End If
Me.TreeView1.SelectedNode.Tag = Me.TextBox1.Text
End Sub