This plugin is delivered with the NopAdvance Core plugin on which this plugin is dependent. You need to install the plugin using the plugin installation guide provided here.

Once the plugin has been installed, you will see the plugin menu under NopAdvance > Plugins > Cancel Order Option in your nopCommerce admin menu, similar to the below image.

Click on the Configure menu item from the Cancel Order Option plugin menu as displayed above and you will be redirected to the plugin Configuration page.