Symptoms:

• On Mac OS X Snow Leopard (10.6.x) or Mac OS X Leopard (10.5.x) you get the error message: “The VPN server could not be found. Verify the server address and try reconnecting.” when trying to connect to an IPSec VPN server.
• You are able to ping your VPN server address
• You have verified that all details entered are correct
• Your Mac OS X firewall is not blocking incoming VPN communications
• You are connecting to the internet from behind a router, very possibly a Linksys or Netgear router.

Explanation:

This error can be caused for many reasons such as incorrect address, firewall or internet connection problems, etc so I’m going to assume that you’ve checked all basic settings first before proceeding with the solution below.

The solution below describes how to open the IPSec Passthrough setting on a Netgear router but you should be able to find the same setting on your own router.

Solution:

1. Open a web browser and enter the address of your routers IP address
   • In my case, this is 192.168.2.1.
   • You can find yours by going to:
     • System Preferences > Network > Airport (or Ethernet depending on your connection type) > Advanced > TCP/IP > Router
2. If you are prompted, enter your username & password
3. On the left hand menu, click WAN Setup
4. In the main pane, click Enable to the right of IPSec Passthrough
   
5. Click Apply
6. Retry connecting to your VPN

Steps 3 & 4 in the above solution apply to a Netgear WNR2000 router.  Please add your router model and instructions for steps 3 &4 to the comments section below in order to help other users with different routers.