In some cases, the PlayStation 3 app will not appear in the PlayStation store itself. Typically, however, if you're in a supported store region, you can still access the app:
- Go to TV/Video Services
- Select My Channels
- The Plex for PlayStation app will be available there
- When you access it for the first time, it will prompt you to download and install the app
Related Page: Where is Plex for PlayStation available?