Follow the guide “Jailbreaking 101 - Seas0nPass” for “Using Seas0nPass to jailbreak the Apple TV” her:
BUT - be aware of a few changes (A and B below):
A) Step 1: Unzip the ‘Seas0nPass.zip’ file and launch Seas0nPass.
Start this step WITH the ATV2 connected only with micro-USB.
B) Step 4: When prompted, connect your Apple TV to your Mac/PC using a micro-USB cable (leave power cable disconnected).
Once you get to this step, connect the power cable to the ATV2 while the micro-USB is still connected. Then pres “menu” and “play” on the remote at the same time until the ATV2 starts flashing. Unpress (let go) the “menu” and “play” buttoms once the ATV2 start to flash, and remove the power cable (ONLY the power cable, leave the micro-USB cable in).
After this you are pretty much done and the guide tells you the rest.