I don’t want to sound patronising but obviously I don’t know what your knowledge level is 
But i2c is basically a wiring standard rather than a communication protocol.
So while many devices can communicate over the i2c bus they require their own specific drivers in order to send and receive the correct information.
It looks like there is Python software you need to install and use, which is designed to communicate with all of the iplate devices.
If you install this and get it working correctly, then you can install the node-red-contrib-pi-plates nodes.
This should allow you to get information to and from the pi-plate devices, but as described in the node info, these make use of underling Python code.
So in short to use the pi-plate kit you do not need to worry about i2c ! (unless you have the skills to write your own drivers for each device)
