Setting up reminders is very simple. You simply have to run the reminders.php script in your /horde/kronolith/scripts/ directory on the shell. To do it manually, simply try this (replace /var/www/horde with the path to your Horde installation):
php /var/www/horde/kronolith/scripts/reminders.php
If this works (you will get an email), you have to create a cronjob to call the reminders skript regularly. In this example, it is called every five minutes. So edit your crontab:
crontab -e
and enter the following (replace /var/www/horde with the path to your Horde installation and /usr/bin/php with the path to your PHP CLI binary):
# Kronolith reminders */5 * * * * /usr/bin/php /var/www/horde/kronolith/scripts/reminders.php