OS X Server: Creating a Netboot Image

I am trying to generate a NetBoot Image on a Mountain Lion Server (10.8.2) and cannot figure out how to download:

"Install Mac OS X Mountain Lion" application from the Apps Store.

I followed the directions in HT4178 and when I use the option key then selecting the Purchases Tab, I do not get "Download" changing to "Install". If I select "Download" it warns me that I already have the most current OS version. What am I doing wrong?