Insert the following lines right above [extra] in /etc/pacman.conf:
[unity7]
SigLevel = Required DatabaseOptional
Server = https://manjaro.github.io/unity-for-manjaro/x86_64/
Install Unity:
sudo sh -c "pacman -Syyu; pacman -S unity-meta"
Switch to LightDM as the default display manager after disabling the current display manager:
sudo systemctl enable --now lightdm
Voila! You should now see Unity in the session list, and be able to log into it.