in your example you are reading an empty datatable(My_Datatable). Use a Read Range-activity where you read the excel-sheet and save it to My_Datatable. Then you have data in there which you can access.
Please see the attached updated the xaml. I have use the “data row” activity to get the values then used the Assign activity and incremented the value to write into cells. I am not sure of xaml perfection since I did not check the xaml. Because my Uipath has some issue to connect to IE. But this would work. Please let me know if it works.