The Howto Install section refers to the AppStore's own howto, which you can find in the left hand menu below the categories.
Short version: Open the Appstore in your Modeler, go to the app and press Download.
As far as I know this widget is supposed to be used with the sprintr portal which will be launched by Mendix at the end of this month.
Thanks, Robert, that I worked out :-)
Would you know if this is supposed to work with 2.5.4?
Ciao Toon