You need to enter a pin number. If you don't know what it is you'll either need a new stereo (may be expensive and you may require a nissan->iso converter) or try to get the pin for the current one using some software (these quite often don't work though). If you know the pin, just press the radio-preset buttons in order (e.g. if the code is 5372, press 1x5, 2x3, 3x7, 4x2 and then hold 5 to enter the code).
Tom, September 2007