I believe that the XML for the button, hosted on DU is
broken... but you can create your own with the following:
Open Google Toolbar's "Manage" option,
Select "Custom Buttons",
Click "Add More...",
Select a button from the Gallery, such as CNN and click the
"Add Button",
close the gallery window,
find the button that you just added and click
"Edit",
Click the "Advanced" link,
A window will open with XML contents of the button,
replace the contents with the following XML content...
<custombuttons
xmlns="http://toolbar.google.com/custombuttons/">
<button>
<search>http://www.google.com/search?q={query}+site%3Awww.democraticunderground.com</search>
<site>http://www.democraticunderground.com/</site>
<title>Democratic Underground</title>
<description>Democratic Underground - The biggest
liberal discussion forum on the Internet</description>
<icon
mode="base64">R0lGODlhEAAQANUtAAAAAAsLCxISEv8AAEhISJubmxoaGpaWlpSUlJGRkZgAADUAACkpKQoWFkVFRQAICEFBQc4AAKurq5mamlRXVyQxMXsAANHR0X19faSkpDI7Ox8pKYSEhDExMYKCgllZWYyMjEVISGBgYK6urlsAAKCiokoAAFdaWvQAACoAAHh4eG1tbV5eXgAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACH5BAEAAC0ALAAAAAAQABAAAAZlwJZwSCwaj8ikcjkUBArMFgaAUBY0lRMgoSIcWgSCRHgApEwA6goganUAHuFmgRooAJOW4SMEcIQNJAMDEQAleix9IEIUABYKCwAOFwwMCQ4AEEMhDwANTiMZAQABAgJRqKmqR0EAOw==</icon>
<feed
refresh-interval="240">http://www.democraticunderground.com/rss/lbn_feed.php</feed>
<update>http://www.democraticunderground.com/du-google-button.xml</update>
<!-- ChromeFoundry - 2006 -->
</button>
</custombuttons>
...
click the "Save to Google Toolbar" button,
Click "replace" button.
Click the "Close Window" button.
Close the Manage window...
The button should now be available in the Google toolbar.
Hope this is what you are looking for.