Download by going over to the Releases section just on the right pane of this webpage. And then open Blender > User Preferences > Add-ons > Install, go to the zip file you've downloaded from this page ...
Mixamo changed the rig of the X-Bot character, which means you need to upload the old rig provided in this file mixamo_old_rig.fbx and use it. You can convert Mixamo rigs / animation to Emoticons rigs ...