How do I enable it?

Look in your freshclam.conf (usually located in /usr/local/etc/freshclam.conf) for the entry “SubmitDetectionStats” and ensure it is enabled and points to your clamd.conf, for example “SubmitDetectionStats /usr/local/etc/clamd.conf”. Then look in your clamd.conf (usually located in the same directory as freshclam.conf) and ensure that you have enabled LogTime (set it to “Yes”) and LogFile (which should point to a place such as “/var/log/clamav/clamd.log”). When you enable SubmitDetectionStats freshclam will check the log file of clamd during each regular database update.

If you run your own private mirror of the ClamAV Virus Database, or if for some reason you don’t use freshclam to update your Virus Database, you can still submit your statistics by regularly running freshclam with the −−submit-stats option.
When this option is used, freshclam will just submit the latest information taken from your log file of clamd without updating the ClamAV Virus Database.