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.
Go to this web page and inspect the image. Somewhere, hidden within it, are the co-ordinates of the cache.
Need a hint to help decipher the coordinates? You might find what you're looking for on the supporting web page.