Have you checked this link in docs with all steps for the configuration?
https://docs.mendix.com/appstore/widgets/calendar#2-basic-configuration
If this is not enough, then download the Calendar.mpr file (test project for this widget)
Link: https://github.com/mendix/testProjects/tree/ea1825221d1534596b802fba30dd2940b8db1b1e
Open this test project and try to understand how they have implemented the widget use in demo project