Scene7 CS Extension - Does not contain valid signature

I need to convert a PDF to an fxg file to upload into Scene7. The documentation says I need Illustrator CS6 and the Scene7 CS Extension.  I installed Illustrator CS6 and downloaded the extension from my Scene7 admin account.  The file name is s7csxs.zxp

When try to install the plugin through Extension manager CS6 I get the following error:

"The extension 'Scene7' does not contain valid signature.  The extension will not be installed."

Im on a MacBook Pro, OSX Yosemite version 10.10.5