how can i get it working?

You either (i)have the wrong code, (ii)the radio has been recoded to a different number or (iii) you aren`t entering the code correctly.
I would suggest a "reboot" by disconnecting battery for 5 minutes ( or fuse), reconnect it , then enter code again.
If still fails to work then ask another question here quoting the MAKE , MODEL + ALL the SERIAL NUMBERS on the radio casing/labels and see if anyone here can check/calculate the original code for you.
If it is different, then it will have to be taken into a Dcode Specialist or Main Dealer....but it won`t be free.
Entry instructions are here http://www.davidsonelectronics.co.uk/code_entry.htm

RegWorts
May 2008