Macs have been 64-bit for eons. What version of Acrobat are you using under what OS? Did you migrate the software or did you re-install?
Offhand it sounds like a permissions issue. Try fixing permissions. If that doesn’t work, try running either from an admin account or temporarily running it from a superuser account [root]. Be sure to disable the root account when finished.
"Macs have been 64-bit for eons."
eons" A week must go by for you real fast. I'm fairly certain an EON is longer than a decade. I'm using Acrobat 9 (created many eons ago). After posting my question more searching revealed that this was just one of many bugs Adobe has chosen to ignore.