How to use paywall in flutter?

I would like to use paywall for Flutter.
Although I read this description (https://github.com/RevenueCat/purchases-flutter/releases/tag/6.3.0-beta.1), I can not understand how to configure MainActivity.


This is my current MainActivity.kt, but I encountered the error: Unresolved reference: revenuecat .

package my_personal.app

import com.revenuecat.purchases_flutter.PurchasesFlutterActivity

// Paywall Support

class MainActivity : PurchasesFlutterActivity() {
}