The "authoring kit" as a piece of software does not exist. It is the source code for 4e, and the ability to save protfolios for any custom game you create.
You can "try out" the Authoring Kit by creating a new game system (Develop -> Create New Game System...) and editing the files that are provided. You just won't be able to save any portfolios with your new game system.
You must manually create and edit the XML files to create your new game system, using the 4E and SW source as your reference and
http://hlkitwiki.wolflair.com/ as your guide. That is part of the "steep learning curve" that Rob mentioned.
Once you get your core system up and running, you can use the normal HL editor to enter object data, but setting up the data model, phases, and procedures is all done with lovingly hand crafted XML.
As for editing the XML, there are many good text editing tools out there. My personal preference is Notepad++, because it works well, and it is free.
I am trying to create a game system that is loosely based upon d20, so having the d20 source files available as a base would be great, but that's not an option as it's Wolf Lair's IP. The Savage Worlds and 4E code files are nice to look at, and really help fill in some blanks, but are ultimately useless for making a d20 type game. I'm going to have to completely re-write and re-engineer the d20 concept from scratch to make my game system.