There are a few things you must know to login to a cisco vpn:
The site IP address or fully qualified hostname, your username and password, either the group username and password or have a certificate.
I know this is a horrid thing to ask a fellow mac user to do but can you login okay from a windows machine?
If so, you maybe able to copy the .pcf file that lives somewhere like (from repressed memory
C:\Program files\Cisco\VPN Client\Profiles\ to your mac and use to client to import it into your list of connections.
Here comes the rub.
Some sites, however, require that the client has a recognised firewall (Zone Alarm, Black ICE, Cisco built in one, none of which run on the Mac). If your site is like this, buy your friendly vpn admin some beer and ask to have a new group called something like MacUsers (and to let you know that group password) without the firewall requirement.
You will just have to reassure him that you're running a BSD variant which has (and you know how to use) a very secure firewall. You also know the need to keep your system up to date. Then edit your connnection and replace the groups username and password.
I am using a cisco vpn, but I'm the admin. mmmawwhahhahah. Trust me the beer works ;-)