After adding the Repo, you must then run:
sudo apt update && sudo apt full-upgrade
This should then install xfce 4.16 on the system. If xfce is not yet installed, running sudo apt install xfce4 should then install xfce 4.16.
The zorinmenulite plugin is found within the Zorin OS repository so it should show up just fine if you are using Zorin OS which comes with the Zorin OS repository already added in your sources.
We disagree. 
But if you prefer gdm3, you can switch back to it, no problem
sudo apt install gdm3
Then follow the configuration prompts in the same way you did for configuring lightdm.





transferred your posting to the pre-existing thread with a similar subject.