Add VPN button is disabled in Debian Wheezy/LXDE

This sounds like your issue. I found it in this articled titled: Gnome Network Manager VPN Tab Disabled ( Greyed out ).

                   

excerpt from the article

The Add tab is greyed out when required plugins are not installed for Gnome NetworkManager. The following plugins should be installed under Debian / Ubuntu Linux:

1. network-manager-openvpn and network-manager-openvpn-gnome - network management framework OpenVPN plugin GNOME GUI
2. network-manager-pptp and network-manager-pptp-gnome - network management framework PPTP plugin GNOME GUI
3. network-manager-strongswan - network management framework strongSwan ipsec vpn plugin
4. network-manager-vpnc and network-manager-vpnc-gnome - network management framework (VPNC plugin GNOME GUI)

To install all of the above plugins use the apt-get command as follows:

  $ sudo apt-get install network-manager-openvpn network-manager-pptp \
        network-manager-pptp-gnome network-manager-vpnc

The following plugins should be installed under RHEL / Fedora / CentOS / Scientific Linux / Red Hat Enterprise Linux desktop systems:

1. NetworkManager-openvpn : NetworkManager VPN plugin for OpenVPN
2. NetworkManager-pptp : NetworkManager VPN plugin for pptp
3. NetworkManager-vpnc : NetworkManager VPN plugin for vpnc

To install all of the above plugins use the yum command as follows:

  $ sudo yum install NetworkManager-vpnc NetworkManager-pptp \
        NetworkManager-openvpn

Now, you can add vpn connection to your system using NetworkManager itself. You may need to restart the NetworkManager as follows:

  $ sudo NetworkManager restart

                   

You have to install the network manager:

sudo aptitude install network-manager-vpnc

As well as the gnome client:

sudo aptitude install network-manager-vpnc-gnome