download deb package from https://www.nomachine.com and install it.
# dpkg -i nomachine_4.2.25_1_amd64.deb
Unity is a little bit slow via remote desktop, so install gnome-session-fallback to use lightweight one rather than Unity.
# apt-get install gnome-session-fallback
reboot the OS
Connect to the Ubuntu from NX client.
You will see login screen.
Slect GNOME Flashback(Metacity) and then log onto the Ubuntu.