After installing networkmanager-openvpn in ArchLinux, I managed to add a VPN connection to the NetworkManager. However when I try to connect, I get this naughty little message:

org.freedesktop.networkmanager.openvpn was not installed

Actually this error message will disappear once you restarted the computer. A quick fix to this is by restarting the NetworkManager through systemd

# sudo systemctl restart NetworkManager

Happy VPN!