How to add field to existing sql report in visual studio 2008

I have an SQL report in a visual studio 2008 project. I need to add a new field to the report but I can't see the data source. If I open the data source pane it is empty. If I run the project the reports runs well. Can anyone suggest a solution? I am new to reporting services.

thanks

ldj

Is the datasource based on a stored procedure? You can check by clicking on the parameter button "..." next to the dataset combo box.