1.

Select an event or To Do item and click the Info button in the bottom-right corner of the iCal window to open the Info drawer.

In Day or Week view, you can also double-click the bar along the top of an event (where the event's start time appears) to open the Info drawer.

2.

To change the alarm, choose a type of alarm from the "alarm" pop-up menu. Then enter your specific alarm settings.

You can only set an alarm for To Do items that have a due date (in the Info drawer). The time of the alarm is based on the To Do "starting" at 12 AM on the due date. For example if your To Do item is due on August 22 and you set the alarm for 8 hours before, you'll receive the alarm notification at 4 PM on August 21.

3.

To add another alarm, click "alarm" and choose Add Alarm from the pop-up menu.

To remove an alarm, click "alarm" and choose Remove Alarm from the pop-up menu.