I just want to get started playing around with the software, but I am confused as to which software I should download. Should I download 'knitml' or should I download the Graphical Pattern Editor and Composer?

If you just want to play around with the tools, download the Graphical Editor and Composer for your operation system. That's a much nicer tool than the core 'knitml' software. The downside is that the graphical tool isn't as configurable as the core tools yet, so if you want to play with a lot of rendering preferences, you need to download the core tool. Eventually those preferences will make their way into the graphical tool.