After playing with Seas0n Pass and reading all the directions/instructions...I too had a lot of issues trying to get the Apple TV2 into DFU mode but I found a solution:
Using my MacBook Pro (10.6.5)
1) Plugged in BOTH power and USB to the Apple TV2
1) Launch Seas0n Pass and click on Create IPSW
2) When prompted to "enter DFU" mode follow the instructions below
-On the remote hold down both Menu and Down arrow (hold until the light begins flashing VERY rapidly). This will restart the device
-Immediately hold down both Menu and Play/Pause until you see the light flashing rapidly again. Release both Menu and Play/Pause once you see the light flashing rapidly again. Seas0n Pass automatically discovered DFU mode
When selecting "Boot Tethered", i used the same "DFU mode" instructions I posted above and successfully completed that step as well.
Just to make sure this wasn't a "one-off" I attempted these steps three more times and it worked every time.