Thank you iamVFX.
As you said I've copied the scripts in the user folder:
but I don't know where to look for those scripts in SI.
So, I opened the Script Editor, loaded the InstantiateNulls, selected the point cloud and then launched the script. At that point the point cloud is deselected but nothing else happens. I've tried with a curve too, but no luck. Am I doing something wrong? Sorry for these basic questions...
InstantiateNulls.py is a script for button, but yes, it's also can be runned through the Script Editor, and there is no need to place it into the plugins folder (unless you wanted to). It's works with selected points only. If you want to create nulls for every point in a point cloud just select them all (Ctrl + A by default) and run the script