Here's a an addition to my previous post on MAC OS X 10.5, Leopard.

I experienced the following error when attempting to use the Cisco VPN Client  (version 4.9.01.0030) across a Verizon L740 Broadband card connection:

Error 51:  Unable to communicate with the VPN subsystem

Please make sure that you have at least one network interface that is currently active and has an IP address and start this application again.

Note:  The Cisco VPN Client (version 4.9.01.0030) worked as expected, without error, across a wired LAN line.

To correct the problem, I downloaded and installed (1) the latest version of the Verizon Access Manager tool, and (2) the Cisco VPN Client version 4.9.01.0080.