After adding plugin with cordova add plugin
command, it's not necessary to include plugin's js file. You only need to include cordova.js (I think you have already done that, but better to clarify):
<script type="text/javascript" charset="utf-8" src="cordova.js"></script>
After adding plugin, you should build or prepare project to get plugin in action. Cordova cli tool will copy the javascript and native (android, ios, etc) codes into platforms/%platform_name%
directory. Plugins will not work in browser mostly, but never in /www
directory (as there should be no cordova.js inside that dir).
I think the problem here is jQuery toggle function that you use. It is actually a function for toggling display state of the element. (see: http://api.jquery.com/toggle/)
This will work when you emulate or run your android app.
<script>
var isKeyboardShown = false;
var softkeyboard = window.cordova.plugins.SoftKeyBoard;
$('#keyboard').click(function() {
(isKeyboardShown = !isKeyboardShown) ? softkeyboard.show() : softkeyboard.hide();
});
</script>