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/
If this works (you will get a 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