Damn tree menus!

So I’m working on an addition to my ImageServer application and I’m hitting a brick wall:
I have nearly 500 possible volumes (representing CD’s) of images and I need users to have the ability to select these volumes for searching using the following nested structure:
DATABASE –> VENDOR –> VOLUME

For instance, imagine the database is “Clipper”, the vendor is “Eyewire” and the volumes are “CD 1″, “CD 3″ and “CD 9.”
There is a YAHOO UI Library tool called “Tree Control” that I’ve been toying with, and I just can’t get it to work for me. Below is a screenshot of their demo in action:

This is making my brain hurt so much. Any takers?

UPDATE:
So I have an update with an XML file for your review: Super_Search.XML. The file shows the relationship of Database to Vendor (vendor is pretty much just the collection of what’s similar in volume names) to Volumes.
Now, I need to convert the XML I have to the multi-select checkbox format that the Yahoo UI toolkit demonstrates.
Any takers?