1. Plug in your ATV 2 with the usb, and run GreenPoison.
2. Plug in the powercord for 3 sec, and unplug.li
3. Hold down menu and play / pause for 7 seconds.
4. When its in DFU, click "Jailbreak"
5. Once its completed, let it stay plugged in for atleast 40 seconds before removing.
6. The first time you start up your ATV 2 after the jailbreak a new menu called Greenp0ison will show up, from here you can install various apps including XBMC.
I am fairly sure you dont get a web browser with just using greenpoison amongst other things ? Plus atv flash is only the beta version so there is more to come.
Is it any easier to set up? Sounds like you have to jailbreak either way -- it won't do it for you. XMBC looks like a standalone media center versus aTV Flash which kind of adds on to what's there. The web browser thing looks cool, and it'd be nice to use my iPad as a keyboard.