Noel - the error message is telling you that your code is trying to reference non-existent fields in the handles structure. So the line
set(handles.DiffFilePath ,'string', DiffFilePath);
will fail since DiffFilePath has not been set as a field within the handles structure. (I'm kind of surprised that the previous line didn't generate the same error.)
When setting fields within the handles, the typical way to go about this is as follows
% set the fields
handles.DiffFile = DiffFile;
handles.DiffFilePath = DiffFilePath;
% save the data
guidata(hObject,handles);
Since handles, within your callback, is a copy of the structure that contains all handles to widgets/controls and user-defined data, we must call guidata to save the updated handles structure. Upon doing so, these new (or updated) fields will be accessible in other callbacks that access the handles structure.
