If you're using an app like PBS Video app for iPads, look for the app menu in the top left corner of the screen, marked with three lines. 

Tap these lines to open the menu. 

Look for a "Sign In" option and sign in with the same Facebook, Google, or PBS account you used to activate your Passport. 

On any device you'll want to sign in, which will give you access to the Passport videos. If you sign in with the wrong account, or you are not signed in, you will instead see a screen asking you to become a Passport member to watch those videos.

Once your video is playing you'll be able to cast it to your TV using the cast icon, typically located in the top right corner of your device.