how can I add the aidl file to Android studio (from the in-app billing example)

Adding this as an answer since it seemed to help quite a few people.

1. Create a new directory named 'aidl' under 'src/main/'. It should look like 'src/main/aidl'.
2. Add a new package name 'com.android.vending.billing' to the directory 'src/main/aidl'
3. Locate your sdk location and go to "sdk\extras\google\play_billing". Default location for the sdk is "C:\Program Files (x86)\Android\android-sdk". If you custom changed it, then you will have to figure out the location through the sdk manager.
4. Copy 'IInAppBillingService.aidl' into the package created above. In the end, it should look similar to the image below.

5. Rebuild project and it should be good to go.

Note: Make sure you include the necessary import if your reference isn't working

import com.android.vending.billing.IInAppBillingService;

https://issuetracker.google.com/issues/36973270

Edit From Comment

After I did this, the references to IInAppBillingService in my code were still highlighted as errors, but after rebuilding the app, the class was recognized

Just as the error message says, you need to put IInAppBillingService.aidl in the correct directory dictated by it's package (com.android.vending.billing).

Within the src/main/aidl/ folder you already have, put the .aidl file in com/android/vending/billing/.