Now, get an NBM file from somewhere, e.g., the Space Invaders plugin, which is here:
Put the NBM file into the folder you created. I.e., now you'll have this:
Now start up NetBeans IDE. You'll see the updater dialog appear and that the folders you created above vanish. The plugin is installed, it is now found in the NetBeans IDE user directory, instead of the installation directory. And then you can play space invaders or whatever you installed.
I.e., the above is a quick trick to install plugins into a NetBeans Platform application, without needing a Plugin Manager or anything else. I learned about this from Ayman Habib in this interview, where he says: "One tip that I found useful (especially because we do not expose the Plugin Manager as of yet) is that we can distribute modules as NBM files and that they get installed automatically if the users put those NBM files in a directory (update/download/) below our “cluster” directory. I have used this a few times to deliver special purpose modules and was impressed by how smoothly that went."