In order to send a reminder email to your participants you must select the 'Email Reminders' tab.
You must then select the 'Send reminder email 24 hours before event'
You will then be given a text field in which you will find a pre-populated message to your participants. This is editable, so feel free to input information which will be relevant to people attending your event.
Once you have done this, don't forget to 'Save Changes' at the bottom of the page. And email will be sent out to those you have signed up to the event 24 hours before it is due to start.