In order to allow Reviewers to download the original proof:
* The download option is only available for an active review
* This option is 'off' by default (via the admin area). It would need to be checked with each proof upload (or edited in the Advanced Info area of the proof info page after the fact).
If you'd like to turn the default to 'on', go to My Account --> Account Settings --> Customize --> Upload Preferences --> Allow Asset Download.