How to add the Plex app to your NOW TV Smart TV box, and stream movies from your laptop to your (non-smart) TV.
See here: https://www.youtube.com/watch?v=lmQBcbVLWZc
Enter developer mode in the NOW Smart Box
First go to your NOW Smart Box, and enter developer mode. At the channel selection screen, press the following sequence of buttons.
Home x 3, Up x 2, right, left, right, left, right. You have to do this fairly quickly but when you get to this screen you have succeeded.
Press the ok button to enable developer mode. Note the Smart Box IP, username and password.
At that point the Now TV will reboot.
Install Plex for Roku app
Get the Plex for Roku app (the NOW Smart Box is just a restricted version of the Roku box). Get the official one from the GitHub repo: https://github.com/plexinc/roku-client-public , or download the ZIP file: https://github.com/downloads/plexinc/roku-client-public/Plex.zip
Go to the Smart Box IP in your laptop browser, and login. Upload the Plex ZIP file to install it.
Plex will now be found in your channels.
Install Plex Media Server
Get the media server app for your laptop/desktop, from plex.tv, and install it.
Configure it to connect to your Plex app on your NOW Smart Box, and now you can stream!