Run in konsole:
martin@martin-laptop:~$ tdenetworkmanager
[TDE NM Backend ERROR] [/build/buildd/tdelibs-trinity-14.0.0-r844/tdecore/networkbackends/network-manager/network-manager.cpp:1699] org.freedesktop.DBus.Error.AccessDenied: Property "Autoconnect" of interface "org.freedesktop.NetworkManager.Device" isn't exported (or may not exist)
[TDE NM Backend ERROR] [/build/buildd/tdelibs-trinity-14.0.0-r844/tdecore/networkbackends/network-manager/network-manager.cpp:1650] Attempting to access the network-manager VPN service returned: org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.NetworkManager.VPN.Plugin was not provided by any .service files

Hmm. I have a file in my home catalogue since I installed network-manager-vpnc called nm-vpnc-service.conf.

It contains:-
<!DOCTYPE busconfig PUBLIC
 "-//freedesktop//DTD D-BUS Bus Configuration 1.0//EN"
 "http://www.freedesktop.org/standards/dbus/1.0/busconfig.dtd">
<busconfig>
        <policy user="root">
                <allow own="org.freedesktop.NetworkManager.vpnc"/>
                <allow send_destination="org.freedesktop.NetworkManager.vpnc"/>
        </policy>
        <policy at_console="true">
                <allow own="org.freedesktop.NetworkManager.vpnc"/>
                <allow send_destination="org.freedesktop.NetworkManager.vpnc"/>
        </policy>
        <policy context="default">
                <deny own="org.freedesktop.NetworkManager.vpnc"/>
                <deny send_destination="org.freedesktop.NetworkManager.vpnc"/>
        </policy>
</busconfig>

Is this relevant?

Looks like yet another network-manager API change; the TDE network manager expects to see the VPN service at "org.freedesktop.NetworkManager.VPN.Plugin", while network-manager is providing it at "org.freedesktop.NetworkManager.vpnc".

Whenever a service uses DBUS I cringe--there are no compile time checks and for some reason most DBUS service authors think they can change the API every single release. :-(

Actually, on closer inspection the network-manager API did not change.  Have you rebooted after installing the network-manager VPN plugins?  According to this Launchpad bug report the VPN DBUS service does not appear on the bus until the network-manager service has been restarted: https://bugs.launchpad.net/ubuntu/+source/network-manager-pptp/+bug/365071

No luck after reboot.

This is a list of the files in /etc/dbus-1

martin@martin-laptop:/etc/dbus-1/system.d$ ls
avahi-dbus.conf               nm-dhcp-client.conf                  org.freedesktop.PolicyKit1.conf
bluetooth.conf                nm-dispatcher.conf                   org.freedesktop.UDisks.conf
com.hp.hplip.conf             nm-vpnc-service.conf                 org.kde.kcontrol.kcmremotewidgets.conf
ConsoleKit.conf               org.freedesktop.colord-sane.conf     org.trinitydesktop.hardwarecontrol.conf
hal.conf                      org.freedesktop.ColorManager.conf    skype.conf
NetworkManager.conf.dpkg-new  org.freedesktop.ModemManager.conf    wpa_supplicant.conf
nm-avahi-autoipd.conf         org.freedesktop.NetworkManager.conf

Any help?

Is there any way to interactively 'talk' to d-bus?

d-feet is the only way I know of to access DBUS services in anything resembling a sane manner.  As the VPN support of tdenetworkmanager is largely untested on NM  >= 0.9 I guess additional fixes are needed.

This should be resolved in GIT hash 73618d0.  Note that you may need to delete and recreate (or at least modify and save) your OpenVPN connection before it starts working.

If you receive an Error 0 or Error 1, try looking in your syslog to see why OpenVPN failed.  Errors 8 and 9 indicate a problem in the connection settings.

Still no-go. Same problem. The CLI utility vpnc works with the appropriate details.
The cisco vpn we use has a group ID and a group password. I do not get prompted for the group password which is required for the initial connection.
All of the needed information is stored in a credentials file for vpnc so all the information is already present.
I do not get prompted by the networkmanager to add any details.

The NetworkManager 0.9 VPN service seems to be broken in general; i.e. in my tests it won't even work if the VPN connection was created with the stock Gnome nm-applet application.  Therefore, I don't know how much (if any!) of this problem is a TDE issue versus a NetworkManager problem.

vpnc bypasses NetworkManager entirely, so I am not surprised that it works.

*** Bug 435 has been marked as a duplicate of this bug. ***