Updating path to an external drive

I recently added Aperture 2 to my iMac, transferred my photo files over, added an outside drive (Western Digital Studio Edition II), and set up a vault in the external drive. Everything ran fine for weeks until a few days ago when I attempted to add to my vault and was told that I needed to "update the vault path". The external drive is still handling all my regular backups, so as far as I can tell the only thing wrong is the corrupted (actually it's completely missing) vault path from Aperture 2. I found the Update Vault Path window in Aperture 2, 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.