I have a Samsung HT-J5500W and coundn't find Plex App. So after a little google search I came across this:
I followed the directions of plexgramm. And I got plex to work.
Basically you have to sign in to https://publish.tvstore.opera.com/ and create an account as if you are a developer.
Next it will ask you for the developer code for your Opera device. I found mine under the Tools option.
Once your device is linked to the Opera website you should see a new tab called "Developer"
Under the Developer tab you have the option for the URL Loader. Type in http://plex.tv/web/tv/opera and log into plex as if it was a new device.
Hope that helps.