I bought an 'N' reg Almera just over 2 months ago and have the same problem. If you are just locked out then you can go on to Ebay and purchase a CD with all the radio codes for around £1 (all you do with this programme is enter the car radio details and barcode and allow it to generate the right code for you). If, like me, you are permanently locked out (because someone was continually punching in the wrong code into your radio) then i have been told that Nissan charge £20 for the pleasure of unlocking. Personally i would rather just put the money towards a decent cd player or just buy a cheapie used radio instead...
Ali G, November 2005