In order to enable periodic tarsnap backups you need
to add these lines to your /etc/periodic.conf

daily_tarsnap_backup_enable="YES"
daily_tarsnap_backup_paths="/"
daily_tarsnap_backup_keep=7
weekly_tarsnap_backup_enable="YES"
weekly_tarsnap_backup_paths="/"
weekly_tarsnap_backup_keep=5
monthly_tarsnap_backup_enable="YES"
monthly_tarsnap_backup_paths="/"
monthly_tarsnap_backup_keep=2