The extension package is invalid. The extension will not be installed.
means that your download of the MXP file is corrupted.
delete the file you downloaded, clear your browsers cache, then download it again. Make sure to select the option to save the file to your computer rather than to open it.
If you still have problems, try downloading using a different web browser.
Also make to turn off all anti virus programs and other programs that may scan files as they are created.