Go to Crystal Report -> Field Explorer -> Runing Total Field -> Right Click -> New -> Give The Name -> Select Field To Summarize From Left Hand Panel(Note :- If you are doing sum of any entity remember that in your database that entity should be in 'INT' Form, Otherwise in Dropdown Menu The Option of sum will not be shown) -> Then Select Type of Summary -> OK -> Again go to Field Explorer -> Expand The Runing Total Field -> u will be shown ur Entity just Drag down on your Crystal Report
wherever you want