Dear Karen,
Thank you for the details.
Could you please check if you have installed Microsoft Visual Studio for office which is pre-requisite.
If so, kindly go to below path and before this close excel.
C:\Program Files (x86)\SAP\Add-In\Add-In for Microsoft Excel 2007
Then double click on VSTO file. In case if there was any issue , it will repair.
Then try to installing excel add-in.
Incase if you still face issue, provide us below details.
a. Operating System.
b. Screenshot of your Office package installed.
Thanks & Regards,
Sanath