Problem with Rg.Plugin.popup plugin

So i've installed this plgin called rg.plugin.popup, and its needed to define it in mainActivity like this

Rg.Plugins.Popup.Popup.Init(this, bundle);

But the error is : "Severity Code Description Project File Line Suppression State
Error CS0234 The type or namespace name 'Popup' does not exist in the namespace 'Rg.Plugins.Popup' (are you missing an assembly reference?) CloudDMS.DemoMobileApp.Android"

Anyone had a similar problem with this plugin?