From the plugin description, it seems like you have to add the file in the plugin directory, under res/values (in AppBuilder Plugins/AndroidInAppBilling/res/values/billing_key_param.xml) and put in it:
where value is your own value.
Another way is to use this plugin:
in terms of settings inside the app are quite easy to follow. To add the preference setting in AppBuilder, right-click the project name and navigate to Edit Configuration
-> Android Config.xml.
There, you can add this preference along with the existing ones.