Active subscribers to Plex Pass have access to many Plex apps for free. Plex for iOS is one of those apps. In this case, you'll simply download the standard Plex for iOS app and then activate it by signing in with your Plex account.
Activate By Signing In
When you launch the Plex for iOS application for the very first time, you'll go through a short "first run" wizard. You can choose the Sign In option at the start of the wizard.
Enter the username or email address as well as the password for a Plex account with an active Plex Pass subscription.
In case you skipped the sign-in during the wizard, you can always access it under Settings > Account at any time.
Related Page: Plex Apps > iOS > Settings