If you just drag and drop the archive with Motion_Node (v.2.0.2) into the Blender window, it will automatically install the addon and write the path to the nodes — you don't need to do anything manually.

However, if you prefer to install the nodes manually, this is also possible. For this:

1. Download the MotionNode archive (only nodes).
2. Unzip it to a convenient location.
3. In Blender, go to Edit = Preferences = File Paths = Assets Libraries and specify the path to the folder with the nodes.

Links:

Documentation | Tutorial | Youtube