I've installed the plugin, while installing the plugin I received the following warning:
"The plugin generated 252 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin."
I had created a folder called subscribe-widget (within wp-content/plugins) & assigned it the permissions 0777.
When configuring the widget in the 'Widgets' section all goes as expected. I click save, check the actual site & do not see the plugin. I do not have a caching plugin & I have reset my cache.