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.
First, please uninstall Express Plugin just like before.
Then manually download and install 2 packages directly from Microsoft's support website. Download both of them one at a time and run the installers.
After installing both of those, restart the computer and then install Express Plugin again.