Try the code below:
string valueByColumn = FlexGrid1.Rows[FlexGrid1.RowSel]["ColumnName"].ToString();
string valueByIndex = FlexGrid1.Rows[FlexGrid1.RowSel].ToString();
For passing column index in second code line, it should start with 1 (not from zero)
. Because the flex grid has been implemented internally in this way. If you pass zero as column index, it will throw exception as Object reference not set to instance of an object.
Hope this helps!