this.LabelControl.Text = datatable.AsEnumerable()
.Sum(x => x.Field<int>("Amount"))
.ToString();
If you want to filter the results:
this.LabelControl.Text = datatable.AsEnumerable()
.Where(y => y.Field<string>("SomeCol") != "foo")
.Sum(x => x.Field<int>("MyColumn") )
.ToString();