Several days after iOS 5 was released, Apple stopped signing previous versions of their firmware, meaning you could only use iTunes to restore your device to the latest software version which cannot yet be jailbroken. Thankfully, there is a work around that allows you to forge the firmware signature to the firmware file before restoring it to your device, allowing you to reflash your device with the same software version that you currently have installed, or to any version for which you have the SHSH Blobs saved.
JAILBREAK THE APPLE TV 2 AFTER APPLE STOPPED SIGNING OLDER FIRMWARE
Works on Windows Only!
Step 1: Save your SHSH Blobs to a safe place. You’ll need them to downgrade or reflash your device in the future.
Step 2: Build Signed Firmware using the SHSH Blobs and the corresponding Factory Firmware file.
Step 3: Patch the Jailbroken Code to the Signed Firmware you just created.
Step 4: Restore Custom Firmware to your device using iTunes.
Congratulations! You’ve now successfully jailbroken your Apple TV 2 device. It would probably be a good idea to Install nitoTV and other software at this point.