In order to set up the alarm using the aCalendar application, you can open the application and choose a day. A new page should appear. Tap on the New event button from the upper right corner of the screen and under REMINDERS you can find the available option for the alarm.
If this feature doesn't work for you, the cause might be an internal error of the application. To fix it, I suggest to update the aCalendar tool to the latest version available.