Where is CurrentAppSimulator Class in Windows.Services.Store namespace?
The CurrentAppSimulator Class comes from the Windows.ApplicationModel.Store namespace. As @IInspectable mentioned, the document had already said that The Windows.ApplicationModel.Store namespace is no longer being updated and
it is recommended that you use the Windows.Services.Store namespace instead.
To test your in-app purchase, you could check the Test your in-app purchase or trial implementation part of the document, which explains how you could test your in-app purchase. You could create a free add-on for the testing purchase.