AFAIK the Brackets plugin isn't really supported anymore. It should still work but using the mendix-widget-generator is much more simpler You need node.js and the widget generator
After you've installed that, doing what Chris said should work: navigate to the root directory with Powershell and type yo mendix The Generator will run (if installed correctly :-) ) and it'll notice you ran the command in an existing widget directory and asks if it should be updated. The generator will automatically run 'grunt build' to build a new widget package and place it in the test project and in the dist folder.
Using the generator is much faster since you can 'grunt' your project directory and it'll automatically watch for changes and recompiles those. So no more copy and pasting the widget.mpk to the right directories :)
Hope this helps!
Put the widget in the src folder and run "yo mendix" in the parent folder. Choose to update an existing widget. It will create the folders for you.