There's a reason I keeping linking to
Wikitravel's Expo 2005 page, and that's because it (hopefully) answers all your practical questions. In your case, the easiest way of getting to the site is the Expo Shuttle train and transfer to bus/LINIMO at the other end. There are
insane amounts of volunteers, guides and baton-wavers-at-large in Nagoya and at the site so just ask if you ever feel even remotely confused...
On with the story!