The hand replayer accepts a single PokerStars hand history, starting from the "PokerStars Game #6189146026: Tournament #31379266...." line.
If you paste your hand history and nothing happens (blank screen, empty table), you probably entered the wrong part of the history text. Everything from the top line (as described above), to the two blank lines at the bottom.
If all goes well, you'll see your hand animated after a short pause, and you'll receive a permanent link to that hand, so you can share it with everyone (if you want to!). You are free to use the hand replayer as much as you want - and please do send us suggestions for new features or bug reports. You can comment on the blog entry if that's easier. Enjoy!