Ohh.. My appologies... You have to group by both prperties to access it, thn you can access it as k.key.Renta, k.key.Category.

I think you have to use LINQ rather than the extension.

var groups = from t in DataSet1.TotalRevenuwByMonth
             where t.totalcategory.Equals(Helpers.TotalCategory.PNEWBUSINESS)
             group t by t.totalcategory into g
             select new { totalcategory= t.totalcategory,
                          productFamily=t.productFamily
                          sum1=g.sum(a=>a.field1),
                               sum2=g.sum(b=>b.field2)};