Open up Dictionary preferences and just simply enable the three dictionaries. You'll see the new tabs at the top of the window. Very useful for me and easily overlooked if you didn't take the time to look!
This is extremely useful for Japanese students or translators since it'll search directly from the contextual menu.
[crarko adds: I looked and they are indeed there. If you did a custom install of OS X and didn't add in the Asian language support I would think they might be missing. An earlier hint inkled at this in 10.5, but the feature may not have been finalized until Snow Leopard.]

