By default, Keychain Access only shows you your own keychain. If you click the expand button (triangle inside a square) at the bottom left corner of the Keychain Access window, you can show other keychains, including the System keychain. Once you're looking at the System keychain, the item you want has a Name matching your VPN, and its Kind is "IPSec XAuth Password". You can find it by sorting by Kind.

An easier alternative (from here) is simply to type "xauth" in the search box at the top right corner of the Keychain Access window. (This works even if you don't have the Keychain list expanded and aren't looking at the System keychain.)

Then you can follow the rest of the instructions above to allow configd to access the password.