Sometimes Excel will associate an add-in with an error that has occurred, and take the precaution of disabling it. This disabling does not simply deactivate it, because if a user tries to reactivate it via one of the add-in managers, it will still not work.
To determine if an add-in has been placed in the disabled items list, go to Office Button | Excel Options | Add-ins:
Notice the item in the "Disabled Application Add-ins" section of the list. To make this add-in available for use again, select "Disabled Items" in the drop down list, and click the Go button.
Excel will then show the "Disabled Items" dialog. Select the add-in you want to enable, and click the "Enable" button.
