Michael,
I would suggest you to perform the system updates (including system security updates). Once you updated your system, perform following steps to enable Microsoft Office Add-in in Microsoft Excel sheet:
1. Open Microsoft Excel application.
2. Select the large icon at the top left hand side of screen (Office tab menu).
3. Select the Excel Options button (found on the lower right hand side of the menu screen)
4. Select Add-Ins > Go (to the right of "manage add-ins")
5. Place a checkmark in the box next to "Microsoft Live Office Add-in" and click OK.
Once you enabled the Add-ins, try to edit your Microsoft Excel sheet again. In case, if you found problem still persisting then let us know following details:
1. Operating system and browser version
2. Microsoft Office version installed on computer.
3. Microsoft Office Live Add-in is installed properly or not.
4. Issue is specific to Excel documents or with all types of documents.
Thanks
Sunil Raina
Microsoft Support Engineer