Head over to https://account.tablotv.com/account/registration/. After you have created an account, the Tablo subscription store will ask you to enter your ENGINE ID.
You can find your Tablo ENGINE ID by navigating to the Tablo Settings screen from the Tablo ENGINE application.
Navigate to Subscription > Subscribe now. Your ENGINE ID will be displayed in the center of the screen. Enter this number on the registration page at https://account.tablotv.com/. Once you have registered your Tablo ENGINE ID, follow the rest of the subscription wizard to select and purchase a subscription.