If you have ever encountered situation, where you need to add In-app billing to your existing project, than this blog is for you. Please note that for this you need Google Play Account and valid Google Merchant Account.
First open Android SDK manager. You can open it from Window - > Android SDK Manager. If it's not available in menu go to Window -> Customize Perspective and Go to third tab Command Groups Availability.
And check the Android SDK and AVD Manager. Now it should be available in menu. Open it and Go to Extra.