Error during install. Unable to install plugin. Microsoft Office Customization Installer.

How to fix installer error: Add certificate to trusted publisher list.
For some users, the Express Addin installer stops with an error "Installing Office customization: The solution cannot be installed because it is signed by a publisher whom you have not yet chosen to trust. If you trust the publisher, add the certificate to the Trusted Publisher list."
The best way to fix this is to setup Office applications to prompt you if you want to continue installing software when the installer is signed with a valid certificate. 
Here is a quick fix to setup Office to prompt you instead of ignoring signed installers.
1. Uninstall Express plugin using the Add/Remove programs wizard from Windows.
2. Download the following ZIP file. Extract the REG file and then right-click on it and choose "Merge". 
3. Reinstall the Express plugin and this time, you will get prompted to allow it to install into Office.
The Registry file that you should merge into your Windows registry adds the following key and settings.
Here is the registry key and values that will be added to the customers’ registry.
For more information about why this is needed, here is a Microsoft Deployment document about updating the Office Click-Once Trust prompt behavior.