cant build IInAppBillingService.java (in-app purchases)

I was doing this following these steps as mentioned in the link.

1. Created directory /src/com/android/vending/billing and copy the IInAppBillingService.aidl file into this directory.

    

2. Rebuilded application. (a generated file IInAppBillingService.java in the /gen directory expected, but nothing!)

when I finished that, IInAppBillingService.java still not exists in \gen

whats wrong?