make sure you don't have a previous version of Free Tools installed before starting to install the newer one.
if you do have a previous version installed, uninstall it from the extension manager.
Also, make sure to save the extension MXP file to your computer. When you download the extension, you may be presented with the option to Open form the current location, or save it. if you are prompted with this option, select the option to save it to your computer.
after the download is complete, open the extension manager, click the install button in the extension manager, then browse to the MXP file you saved.