My guess is its an "inside post" mount, meaning the main mounting bracket mounts on interior side of the post, not on the outside like a lot of them. You need to contact Unity and they will get you the instructions for that particular mounting kit. You dont want to try and do it blindly. Be patient and get the templates you need. Not worth screwing something up. Because when installing those spotlights, you only have one chance to get it right.
Here is the inside post mounting instructions for you to review, if that is indeed how it mounts. I dont know for sure. I couldnt find the instructions for the mounting kit you need.
http://shop.unityusa.com/assets/images/4392REVGINSIDEPOSTSPOTLIGHTINSTALLATIONINSTRUCTIONS.pdf