1. I don't have a Mac. The key point is that you have to unzip the ZIP file (used because this forum only allows ZIP attachment) to get the TGZ file inside. Some unpackers also unpack the tgz, which results in a usr/etc/mnt directory structure to appear, which is not what you want (and thus you have to tell your unpacker to stop unpacking contents also). But if you got the TGZ file then all is OK.
2./3. You don't have to rename to screensaver.cfg. The mod installs as .example.cfg so as to not overwrite a screensaver.cfg you previously created. If the scanline works, the mod will create the screensaver.cfg on its own.
As for why it doesn't work for you: I don't know.
If you already have standby= poweroff= lines in your screensaver.cfg, you have to remove those for the addon to re-detect them.
Do you have an affiliate that displays a logo on standby/poweroff screens? Those screens are supposed to be entirely blank and only show Poweroff/Sleeping message.
Do you have a charger/USB cable connected to the Kobo? It does not work while charging (adds a battery % message to the sleep screen which this addon can't handle)
Instead of using the power button, try to let the device drop into standby and poweroff by itself (timeout setting in power saving).
Are you using the default Kobo firmware or alternative software like koreader? This addon is meant for the default reader (nickel) only.