There you must select the scope where you want set up the extension, you have 3 scopes, default, website and store views.

Please consider how the scopes and the hierarchy works, the program first check if the store view use an own configuration if it's true the program will use that particular configuration if the store have set [use website], the program will check the configuration settings of the website, if it have an own configuration the program will use that particular configuration if the website have set [use Default] the program will use the configuration settings of the default scope, if the default scope have not the extension enabled even if you have the ecommerce enabled it won't work because in the extension you can choose between enable the list and the ecommerce or only enable the list, but you can't choose enable only the ecommerce, it's because the ecommerce create a store and the extension create a link between the store and the list.

Set up the default scope is useful if you want use the same configuration settings for two or more of your domains (store, list and so on) or if you are using only one domain, if you want use different lists for each domain you should set up each scope (store view or website) with their own configuration settings.