As per Mathias's statement, you just be good to just download it on your computer, whether or not you're already licensed. Then, if you like it, you can buy it. Mild caveat to Mathias's comment, I barely ever used a saved character in developing Planet Mercenary, although I do have some saved files. A lot of it depends on how complicated the system is. Eventually, you will want to be able to actually use your system, so you'll want a license, but it's not an easy process, so you may want to demo it at first.
I don't remember what documentation there is when you load the Authoring Kit, but there's a wiki at
http://hlkitwiki.wolflair.com/index....itle=Main_Page that includes creation of the Savage Worlds system at
http://hlkitwiki.wolflair.com/index....s_Walk-Through. The documentation is not complete (and probably never will be), but there are a lot of examples out there, and Mathias is very helpful.