First, make sure that the plugin is compatible with your computer. You need to be on a PC running Windows XP or newer and have Office 2007 or newer.
If you meet those requirements, try uninstalling the plugin using the Windows Add/Remove Programs wizard.
Open Add or Remove Programs from Windows and find Express Plugin for PowerPoint and uninstall it.
Next, reboot the computer.
If the plugin still does not show up in PowerPoint, maybe there is a prerequisite that is not installing properly. Please uninstall Express Plugin just like before. Then go to the following link on Microsoft's website to download the prerequisite directly. https://www.microsoft.com/en-us/download/details.aspx?id=3508
The Express Plugin requires Microsoft Primary Interop Assemblies for Office 2010 to be installed.
After that finishes installing, install Express Plugin again.