This cache series is not designed to be easy. The caches themselves are all quite easy to find, but determining the coordinates of each will require some puzzle-solving effort.
When you visit the cache, be sure to take a printed copy of the bonus puzzle sheet [PDF] so that you can decipher the coordinates of the secret bonus cache! Alert: You are about to download a file that contains further details needed to find the secret bonus geocache. As the cache owner, I represent that this file is safe to download although it has not been checked by Groundspeak or by the reviewer for possible malicious content. Download this file at your own risk.
The image below looks meaningless, but if you look at it the right way then you'll see that it clearly contains the coordinates of the cache.
Can't see it? Perhaps you should try take a step back and try again. For more tips, see the supporting web page for the cache series.
Note that stealth may be required, and it's not possible to park at the cache owing to parking restrictions (however, there's a pay-and-display car park very nearby.