I am not sure why your structured keywords list would be missing. Are you missing any other saved preferences such as metadata snapshots, Ingest settings, or options you have set in Photo Mechanic's Preferences window?
If you make regular backups of your computer, your structured keywords are located at the following path:
~/Library/Preferences/com.camerabits.PhotoMechanic/StructuredKeywords_UTF-8.txt
Restore that file from your back up (best to make sure Photo Mechanic is not running at the time) and it will restore your keywords.