Error in Adding the In-app Billing Library

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

1. Copy the IInAppBillingService.aidl file to your Android project.
   (1) If you are using Eclipse: Import the IInAppBillingService.aidl file into your /src directory.
   (2) If you are developing in a non-Eclipse environment: Create the following directory /src/com/android/vending/billing and copy the IInAppBillingService.aidl file into this directory.

2. Build your application. You should see a generated file named IInAppBillingService.java in the /gen directory of your project.
3. Add the helper classes from the /util directory of the TrivialDrive sample to your project. Remember to change the package name declarations in those files accordingly so that your project compiles correctly.

But when I finished that, Eclipse gives me an error:

interface IInAppBillingService should be declared in a file called com\android\vending\billing\IInAppBillingService.aidl.

the aidl file is in the correct directory, but the IInAppBillingService.java file was not generated.

Has anybody seen that before?