I am not positive, but I think balloons are not possible if you:
1) Link instead of import the data (maybe I'm wrong and it's just not editable--sorry no time to check myself)
2) You show the data in a form that doesn't have a 1-1 correspondence to the imported records--for example you aggregate the data using the "combine value by" option or use the "show data by every geography" option. In these cases I think balloons are not available because they can only show the actual contents of an imported record. (I think.)