Starting the VPN Client
This is a two step process. In the first step, you will need to start the VPN service by typing the following line (as root):
/System/Library/StartupItems/CiscoVPN/CiscoVPN start
This will load the cisco_ipsec kernel module.
Making the Connection
To establish a secure VPN connection, type
vpnclient <profile name>
If you profile name is profile.pcf, just type profile (leave off the extension). This should start your connection.
Using the CLI Client
You will need to leave the terminal window open (as far as I know) to stay connected. To put the VPN Client process in the background, press ctrl-z. To disconnect from the VPN, you can either kill the process manually or type vpnclient disconnect (after pressing ctrl+z).
For other options, type vpnclient and a list of commands will be displayed:
Cisco Systems VPN Client Version 4.9.01 (0100) Copyright (C) 1998-2006 Cisco Systems, Inc. All Rights Reserved. Client Type(s): Mac OS X Usage: vpnclient connect <profile> [user <username>] [eraseuserpwd | pwd <password>] [nocertpwd] vpnclient disconnect vpnclient stat [reset] [traffic] [tunnel] [route] [repeat] vpnclient notify vpnclient verify [autoinitconfig] vpnclient autoinit