RyS Microsoft DNN Translator -- strange handling by Studio

I have installed this app, which is confirmed by the SDL Plugin Management function:

But I cannot select it with Add, since it does not appear there. Which in turn is confirmed by the Plug-ins list on the Add-Ins ribbon:

So what is going on here? Why does these two features show different results? And why should I not be able to use this plugin when I have installed it?