You can follow this guide to jailbreak Apple TV 2G running iOS 4.2.1 using Greenpois0n, this guide is for Windows users, but you also can ...

You can follow this guide to jailbreak Apple TV 2G running iOS 4.2.1 using Greenpois0n, this guide is for Windows users, but you also can follow the same guide on your Mac OS X.

Step 1:
Download Greenpois0n for Windows, and then open it (download links at the end).

When asked if you are jailbreaking the Apple TV click Yes.

Step 2:
Disconnect your Apple TV from the power and USB then click on "Prepare to Jailbreak (DFU)" button

Step 3:
Follow the next simple steps to place your Apple TV into DFU mode.

Connect the USB cable immediately.

After 7 seconds connect the power cable.

Press the Menu and Play/Pause buttons for 7 seconds

Release the buttons.

Step 4:
Once your Apple TV is successfully in DFU press the "Jailbreak" button to begin jailbreaking.

Step 5:
Once the jailbreak is complete you can press the "Complete" button. 

Important Note: Make sure to wait 40 seconds before disconnecting the USB cable so the jailbreak process can finish up on the device.

Download Greenpois0n RC6.1 Windows
Download Greenpois0n RC6.1 Mac
Download iOS 4.2.1 for iPhone, iPod touch and iPad