The instructions for Seas0nPass to enter DFU mode is
Step 4: When prompted, connect your Apple TV to your Mac/PC using a micro-USB cable (leave power cable disconnected).
Once the light on the front of the Apple TV begins to flash rapidly, point the remote at the Apple TV and hold both the MENU and PLAY/PAUSE buttons for 7 seven seconds.
However, this never worked for me. Not the first time, and not the second time either (months later, when I was updating to AppleTV software version 5.2). The Seas0nPass jailbreak software tool just kept saying “Waiting for device to enter DFU mode…”.
Here’s how I fixed it, replace step 4 above with this:
Step 4: When prompted, connect your Apple TV to your Mac/PC using a micro-USB cable first (and then connect power cable also). Give it a few seconds, then hold the MENU and PLAY/PAUSE buttons on the silver remove (white remote doesn’t work) for about 7 seconds, until the LED starts flashing rapidly. Now the Seas0nPass jailbreak tool will find the AppleTV in DFU mode and continue the process.
Some also reports that it starts working if you try a different USB cable, and still follow the original instructions.
Afterwards, Seas0nPass will run a script to have iTunes update the jailbroken firmware. It will run for a while and eventually my iTunes said that it couldn’t connect to the AppleTV. I removed the power cable (but kept USB cable in), and after a moment the LED started flashing (not as rapidly as when entering DFU mode), and iTunes said AppleTV had been restored.
Now I connected it to my TV and booted it up, and saw that the Settings icon had been replaced with a red image with “fC” in it (for “fireCore”, who’s behind Seas0nPass). Jailbreak completed and successful, now onto installing “aTV Flash (black)”.
All of the above was done on a MacBook Pro Retina running Mac OS X 10.8.2. Your mileage may vary.