Thank you for your question.
Firstly, the external air kit should read external air adaptor, I've changed this in the description now, thanks for bring it to our attention.
To use the external air adaptor the stove would have to be connected to a tube running to a vent in an external wall. this could be in the room and boxed in or could go under the floor if you have a void and it isn't a solid floor.
If this isn't suitable you would need to have an air brick / air vent fitted in an external wall in the same room as the stove.
This must be 550mm2 per kW of stove output over 5kW. So for a Charnwood Island 3 you would need an airbrick with an area of 3850mm2
See more in our article Why do I need an Air vent? https://www.directstoves.com/knowledge-base/home/why-do-i-need-an-air-vent.html
Hope this helps?