Aperture 2: updating vault path

I recently added Aperture 2 to my iMac, laboriously transferred all my photo files over, added an outside drive (Western Digital Studio Edition II), and set up a vault. Everything ran fine until a few days ago when I went to update my vault and told that I needed to "update the vault path". I found the Update Vault Path window, but have no idea how to go any further. Can anyone advise me on how to proceed? I feel like a dolt since it appears that the procedure is supposed to be so obvious that instructions aren't needed.