If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Sometimes the game is won in penalty shots. Make that your goal, and not your opponent's!
As a Striker:
Hit Space to set the direction, height, and power of your shot.
Click to move your goalkeeper to the spot in the goal where the X appears to block the next shot.
Score against your opponent and block their penalty shots to move on to the next round!