Here is our DataGrid sample modified to show
engine.AddNamedRange("TestVal", "SUM(C1:C3)"); //<<<<<<< added
when you type testval into the Textbox and also when you type =testval cell A1.
The only thing I see different than the code you listed is that
engine.RegisterGridAsSheet("SingleGrid", this.dataGrid1, sheetfamilyID);
is in the sample and not in the code you showed.
CS.zip