Unfortunately, there is no information on the Internet that can assure you Microsoft Office 2007 Custom UI Editor works with Windows 8.1. Before, getting in touch with the developer and ask him about the compatibility, I recommend you to try the Windows Compatibility Mode. Right click on its executable file and select Properties > Compatibility tab > and select a previous version of Windows that you know it works. If this method doesn't work, then you can get in touch with the developer.
