You might want to check out his other post: http://www.farinspace.com/show-hide-meta-box-by-category/
I figure you would need some javascript because if you change categories, you would need some way for the browser to trigger to show the metabox, hence javascript.
He has a video tutorial, so that should help explain everything. Also, since you are already reading his blog, you might want to try out his WPAlchemy_MetaBox Class. I use it every time in my themes and I love it.