By Amber Mertes
With Office 2013, the Add-In Designer (msaddndr.dll) was removed.
As a workaround, you may reinstall the VB6 solution for the add-in after you install Office 2013 (if the developer's package included this DLL).
Refer to "Changes in Office 2013" - http://technet.microsoft.com/en-us/library/cc178954.aspx
More information from the Office Development Team:
Support Note: The .dll was removed intentionally, so while continuing to use it is an option, it is not supported. The true recommendation from Microsoft is that developers migrate their code to supported technologies such as Apps for Office or VSTO.
A custom add-in that uses interfaces in the Msaddndr.dll file does not work in Office 2013