You should be using this example:
The previous service which was used by InAppBilling3 library is being deprecated and will soon won't be available. GooglePlayBilling is based on the new in app purchases service: https://developer.android.com/google/play/billing/billing_library_overview Usage instructions: 1. Add to the...
www.b4x.com
Which has two downloads - did you install both?
Also, that error indicates that the missing file is an internal lib which should exist if B4A was installed correctly, so what version of B4A are you using?