Why is my json file not found?

Have you tried removing the ~ ?

As in:

$.getJSON('/Content/dumboJr.json', function (data) {
    $.each(data, function(i, dataPoint) {
        // Bla
      });
  });
)

To allow the IIS to serve JSON files, try adding this to your web.config:

<staticContent>
    <mimeMap fileExtension=".json" mimeType="application/json" />
</staticContent>