How to record data in a SimpleDataField

I want to record data in a Simple Data Field.

I tried to create a field:

using Toybox.FitContributor;
class DataField extends WatchUi.SimpleDataField {
  var myField = null;

  function initialize() {
    myField = createField(
      "my field name",
      0,
      FitContributor.DATA_TYPE_FLOAT,
      {:mesgType=>Fit.MESG_TYPE_RECORD, :units=>"my unit"}
    );
  }

}

But I get a an ERROR: "Undefined symbol "Fit" detected" In line "myField = createField("

What did I wrong?