If you are using the PBS Video app for iPad, you can sign in to your Passport account from the Settings menu at the top of the app when you first open the app. Look for the "Sign In" option and sign in with the Facebook, Google, or PBS account you tied to Passport during activation.
If you are using the Safari browser to go to pbs.org on your iPad, look for the Sign In option in the top left corner. You may have to open our menu by clicking the three horizontal lines icon in the same corner.