If you would like to have the Messaging feature available in your event app, you will first have to add the Messaging feature. You may add the module at the Layout tab.
If you are on the Standard plan, you will need to upgrade the event plan to either the Pro or Premium plan. There are two ways to do so:
1. At Event Details in the Setup tab, select either the Pro or Premium event plan.
2. At the Layout tab, click 'Add New Feature', and click on Upgrade at the Messaging feature.
If you are on the Pro or Premium plan, simply add the Messaging feature at the Layout tab.