On the top right corner of the iPad screen, swipe down to open the Control Panel.
Tap the orientation lock to turn it off (it should be gray/white, not red/white).
When choosing something to watch, the app will display in portrait mode, but once your content is tapped and starts playing, the screen will automatically switch to landscape mode depending on how the iPad is being held.
Once it is in landscape mode, you may turn on the orientation lock again.