Silverlight NumericUpDown and BusyIndicator dlls missing

If you are facing the messages

The type or namespace name 'NumericUpDown' could not be found (are you missing a using directive or an assembly reference?)

or

The type or namespace name 'BusyIndicator' could not be found (are you missing a using directive or an assembly reference?)

You are likely missing the installation of Silverlight's toolkit. Install the one that matches the silverlight version you are using.