Tap the gear icon in the top left of the main view to enter settings, then tap the “Today Widget” row. From here you can add, delete, and rearrange actions.
To enable the Today widget in Notification Center, swipe down from the top of the screen to show Notification Center. Tap “Today” button at the top to switch from Notifications. Scroll to the bottom and tap the “Edit” button. Finally, tap the green plus next to Launch Center.