Oh, in the Tutorial http://www.arduino.cc/en/Tutorial/KeyboardReprogram , with came with IDE 1.5.1 it's:
Keyboard.press('n');
delay(100);
Keyboard.releaseAll();
The Point is, not to send a Key but an condition (pressed or not pressed, that's the question XD )
In the Tutorial a delay ist used, for the game use Keyboard.press when it's pressed and then Keyboard.releaseAll when it's no longer pressed.