The above answers concentrate on file location, but it appears you already had that set correctly. I experienced this same issue in Android Studio, but none of the listed answers resolved it, and I banged my head against it for an hour. Eventually, I realized that I was missing an obvious import:
import com.android.vending.billing.IInAppBillingService;
Once I added that it resolved this error message.
This import isn't mentioned in any of the Google Billing docs or included in any of the code examples that I found. While it may be obvious to experienced Java developers, beginners just trying to learn their first project may need it explicitly pointed out.