Usually when you can't make changes to reminder settings for an event it's because that event is in a category for which you do not receive reminders. Look in the "define sets" dialog and check to see if the various categories have a small alarm clock next to them, signifying that they are set to receive reminders. Pay specific attention to the categories used as defaults for your new events as set up in the preferences.
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Sean Boiarski
Product Manager, Now Software