If you are not hip to pfSense I suggest you check it out. It is an open source firewall that is making waves in InfoSec. The pfSense guys have a great howto for configuring IPSec VPN on the pfSense firewall as well as making connections via a freeware Windows IPSec VPN client called Shrew Soft which can be read by clicking here. The only item lacking in the article is a recommendation for a Mac OSX client as well as configuration tips for a Mac OSX client which is the sole point of this article. Below is information about where to download a freeware Mac OSX IPSec VPN client and then the necessary configuration to make a connection to the suggested settings noted in the howto of the pfSense web site.
Windows XP and Vista both include a L2TP IPSEC capable VPN client. These clients are natively able to transverse client side Network Address Translation. However when the VPN server is also behind NAT these clients will not work without a registry change.