Dear Helen,
To do that, you can open the first file (file A) in the Grid Editor (simply double-click on the file), and then go to Tools \ Copy file into data...
This will open a new dialog, in which you can select the other file (file B) containing the values to be added.
In the 'Item mapping' options, you can select which item from file B has to be added to each item from file A.
In the 'Operation' options, select '+' to sum the data files.
Best regards,
Mathieu