This post is a corollary to the previous one on building iOS client based IPSec VPN with the Stonesoft NGFW.
Testing the same configuration with OS X native VPN client, which you can configure in System Preferences – Network, I’ve found that things have changed a little bit in Mountain Lion (and Mavericks). Due to the Gatekeeper enhanced protection, you need to enter your Keychain on the Mac to modify some privileges.
Click on System and identify the certificate you are using to authenticate your machine.
Once found, you just need to expand it to show the private key portion as shown below (yeah, screenshot is in Italian but I think you get the point):
Then you “open” the access to the component to every application as shown below (again, screenshot in Italian):
Once you do this and confirm, the VPN will restart to work as previously (for example, in Snow Leopard).