Additional Seen Codes were used to specify procedure codes that count as a "Visit" for ONLY the now retired Patient Reminder Measure. This folder should no longer be used (as of 2015).
For offices running builds of MacPractice older than 11.17, you can add an Additional Office Visit Code by going to References > Additional Office Visit in the sidebar.
To all other offices on the newest build (11.17 and newer), go to the Reports ability > Additional Office Visit node in the sidebar.
Although this folder is no longer used, it is worth noting that new Additional Seen Codes can be added by clicking on the green plus above the sidebar.