This looks like a technical issue, as normally it should be like this:
1. You open a workbook.
2. You press "Launch Excel" in MF Pro.
3. It asks you whether you want to connect to the existing sheet.
4. You press "Yes" and it does connect to it.
Obviously something's wrong. When you press "Launch Excel", does it create a new sheet in Excel 2013 as well? (I'm asking because sometimes older versions can still be installed).