It states the following in the manual:

How to set reminders for activities?

Ans: While creating event/task you can set the reminder in the range of 15 min, 30 min, 1 hr ... 12 hrs, ... 1 day .. 6 days, 1 week ... 3 weeks. To set reminders follow these simple steps below:

1. Select a task or an event
2. Click Edit in Event/Task details page and check Remind at and click Save.

Note: Reminder function is available only in the paid version.

However, I can't seem to find the reminder option....