I would like to do that but as I have said, it is on a local server on my PC (Bitnami). Furthermore, I tried again on a different installation and it worked in the beginning (I could see and use the Tag field). After a while, all the articles form the chosen category started to show regardless of the tags I hade chosen. To make thing sworse, when I tried to edit the Addon, the tag field disappeared again.
I guess, I will have to try it on a live server and see what happens.
Sorry I cannot be of any more help and thanks for answering my questions.