Press the P key to start or pause the replay. If you press P and the replay doesn't start, either there isn't anything to replay (that is, the game was never started) or you're at the end of the replay.
There are replay controls at the top of the info box at the bottom of the screen in the 2D map view. Also, you control the speed of the replay the same way you do in the game, the + and - keys on your numpad.
Hook