When the app first opens up, you should see the "My Plans" screen. At the bottom (without scrolling), there should be a blue circle with a plus sign in it (if you don't see it, scroll down and then scroll back up and it should appear). Tap the circle and five smaller circles will pop out around it (L to R: FastPass+, Tickets, Dining, Add Plans, and Shopping). Tap the second from the right - it's a little magic hat symbol - and you'll be taken to a screen that allows you to add Parades, Fireworks, Shows, or Notes to your plan. The Notes function allows you to enter a description, details, date, location, and time.
ETA: It is definitely not intuitive, so don't feel bad for not finding it! I only found it by tapping randomly to see what would happen.