I am sorry for the delayed reply.
Notice that our forum response time is 72 hours and if you really need a faster answer from our side it would be best to open a private ticket
Indeed, custom plugins are not supported in the AppBuilder companion app and Simulator so your option is to deploy your app on actual device where it can be tested. Please refer to our documentation
where all deployment on device options are described.
If there is something unclear please let me know so I can assist you further.
I am not aware of the plugin you are trying to test, but I would say that it is not a part of the core plugins maybe because it is not so popular. We imported basic plugins that are highly demanded by our users in our IDE just to ease their utilization as most of them are frequently used in apps.