Media Center PC > Apple > Apple TV
That's where I found it in the Logitech software. The ATV2 is really nice with the Harmony One. On the little Apple aluminum remote you have to hold down left or right for a couple of seconds for the 30 second skip forward and 10 second skip back feature the ATV2 offers.
But on the Harmony (or really any extra remote you have), you can do it using discrete buttons. I enabled mine using the middle up/down button and it works very well that way. Even better than most DVRs.