The file of rivet doesnt have any . py script attached, so there is no IK, FK system , Where can i get it to be able to follow follow the Tutorial.

If the .py file is isn’t with the download it is actually inside the Blender file itself.

You can ‘save as’ in the text editor, put it somewhere logincal and then install it.  Or just run the file from the text editor when you append the rig to your animation file.