[WPF and VB.net] load <document> node from XML into documentviewer control upon selecteditemchanged in treeview?

Question

This is driving me absolutely nuts.

I've got a WPF browser application that is somewhat like a content management system. We're going to use it for internal employees to get troubleshooting documentation. It consists of a Treeview, to hold all the topics, and a documentviewer control to display XPS documents.

Currently, I have XML that looks as follows:

<?xml version="1.0" encoding="UTF-8"?>
<TopicsList>
  <Product>
    <name>SR-650-101</name>   
    <type>
      <name>Cannot pass POST</name>
      <document>somefile1.xps</document>
    </type>
    <type>
      <name>Serial # shows 12345</name>
      <document>somedocument2.xps</document>
    </type>
  </Product>
  <Product>
    <name>TAV-PDK</name>   
    <type>
      <name>C17200</name>
      <document>somefile.xps</document>
    </type>
  </Product>
 </TopicsList> 

I've got the treeview control to successfully read the xml file, using the following XAML:

<Page.Resources>
    <HierarchicalDataTemplate DataType="Product" ItemsSource="{Binding XPath=type}">
        <TextBlock FontWeight="Bold" FontSize="12" Text="{Binding XPath=name}" />
    </HierarchicalDataTemplate>
    <DataTemplate DataType="type">
        <TextBlock Text="{Binding XPath=name}" />
    </DataTemplate>
    <XmlDataProvider x:Key="TopicsList" Source="topics.xml" XPath="TopicsList" />
</Page.Resources>

and then the treeview being defined as:

<TreeView Margin="0,20,0,0" Name="TreeView1" ItemsSource="{Binding Source={StaticResource TopicsList}, XPath=child::*}" SelectedItemChanged="OnNewNodeSelected" />

Like I said, it correctly lists my "Table of Contents" just fine, so this gets me 95% there. What I'm trying to figure out now, and can't for the life of me, is how to I get it so that when a user clicks a subject in the tree, it will go and get the node from the xml file and use it to load into the documentviewer?

Like I said, everything else so far works great, just cannot find anything online on how to get this to load. I did find something that looks like it would've worked perfectly if I was using a listbox, here however nothing seems to work so far in WPF with the treeview, and I'm just asking for one tiny little thing.

Can anyone help?

Thanks in advance!