Run the following command in your Node-RED user directory - typically
npm install node-red-contrib-pi-plates
First, make sure the Pi-Plates stack is working with the standard python interface by following the Getting Started setup steps here. Note: this package requires python 3.x provided by the 'python3' command, so follow all the python3 instructions for the Pi Plates library and ignore the 2.x variants. The node actually uses a python script under the hood (similar to the stock gpio node) so if the Pi Plates python interface isn't setup and working, then this will not work either.