If its a legitimate unlock code, normally you put a different SIM card in the phone, it will say something like invalid SIM and prompt for a code to be entered. You put the code in, and if all is good, the sim will magically work.

I have no idea what form the unlock code takes, so can't say whether it is a 15 digit code (sounds like the imei number?)

Be aware that you only have a few chances to get it right...

Good luck