When you do an export you have to give the data to the template. You probably defined the template to show PersonActivity data. Before creating the template you then need to retrieve the right data and pass that to the template.
Still not clear. The XLSReport.GenerateExcelDoc javacall has a third parameter inputobject ( any object ).
The inputobject is filled in and I filled in a constraints and sorting in the template, so that should generate in my opinion the data of the excel sheet. In the documentation there is nothing told about retrieving data first.
I think what Ronald meant is that you have to do a retrieve of the object before the call 'GenerateExcelDoc' action. This leads me to another question. What if I want to do more reports on different objects? You have to retrieve the object before you export it. Isn't it possible to use the InputObject you have defined in your template?