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
Note: Your php executable should be the CLI version, it might not work with the CGI version. Do php -v to verify that you have the CLI version.
Using the CGI version will output http header information but does appear to work. You will want to add "> /dev/null" to the crontab entry which also means that you will miss any errors so using the CLI version is recommended.
If this works (you will get an email), you have to create a cronjob to call the reminders script 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