If the source data is not a table, just click within the data list, and use Ctrl-T to make it a table. Then what Norie said will work, just adding data at the bottom of the table and refreshing the pivot table.
Microsoft Excel topics include formulas, formatting, VBA macros and user-defined functions, and everything else related to the spreadsheet user interface, including error messages.
If it is then you should be able to add/delete/change the data in the table and then right click the pivot table and select Refresh.