make sure you are saving the extension file to your system, and using the install button in the extension manager to start the install rather than opening the extension file in the browsers or double clicking on the extension to start the install.
When you download the extension you may have the option to open it, do not select that option, select the option to save it.
after the extension is saved, go to the start menu and right click the extension manager short cut, then select run as Admin.
in the extension manager, click the install button and browse to the MXP file you downloaded.