Message from Uber support:
In order to sign up for the flat fares packages, you'll need to purchase the fares in your app. You'll then have an option in your menu in the app. - it says "Get Flat Fares". Then you'll be taken to a screen that guides you through the purchase. When you've completed your purchase, you will be able to request rides as normal and only be charged the pre-determined flat fare. These fares do not surge, and the number of trips you can get at these pre-determined flat fares depends on which package you purchased.
Not sure how to get the tab back - talking with uber support, will keep you all updated