Solution: There are four ways to view your movie rental credits balance on Mac, iPad, and iPhone. Each method is different, and there is no specific page where the balance is listed as of the time of this post.
Option 1 (Mac App Store app): Launch App Store app on Mac, view the bottom left corner of the window, and immediately under your name, it should state "X Movie Rentals" showing the number of rentals you have available. See screenshot below for an image of App Store app on Mac and where to see your movie rental credits balance.
Option 2 (iPad App Store app): Launch App Store app on iPad, tap your Account icon in the top right of screen, and immediately below your Account ID, you will see "Credit: X Movie Rentals". (This is immediately above the Purchased, Subscriptions, Notifications rows).
Option 3 (iPad iTunes Store app): Launch iTunes Store app on iPad. Scroll to the bottom of the screen. At the bottom left of the screen, you will see "X Movie Rentals Credit" immediately below your Apple ID.
Option 4 (iPhone iTunes Store app): Launch iTunes Store app on iPhone. Scroll to the bottom of the screen. Tap your Apple ID. Tap "View Apple ID". You will see "Balance: X Movie Rentals" immediately above "Add Money to Account".
Apple TV option: No option found.
If anyone knows of a single balances page that lists monetary and promotional credits, expirations, etc, please share. I am requesting that Apple provide a simple universal solution available on all platforms in the same place, so this is less difficult to figure out for users.