Mac OSX Server has the VPN GUI, but what if you have the client / desktop version of Lion or Snow Leopard.
Fortunately, help is at hand, from netputing.com, where there ther is an app called vpn-activator.
Download, drop it into the Applications folder, put in some details, and click 'start'. Easy as.
image from netputing.com
You'll need to setup your firewall / NAT device to forward UDP ports 500, 1701 and 4500 to the machine you want to connect to: