If the FM transmitter not visible in ur phone then the first thing u should do id to check the legal status of your country as most of the countries allow it now

u can check it at  http://europe.nokia.com/fmtransmitter

If the legal status of your country is YES then check your firmware and update it to latest one using Nokia software updater or from nokia care and remember to restart and then changing to factory settings.

If the legal status is NO then u should not try changing the product code because it will void the warranty

In case after updating the firmware the FM transmitter does not appear in music folder then try the following:

press *#0000# and then in options check for updates

for this u should have working Wi-Fi connection or GPRS plan with your operator

After this it will surely appear as the phone is already having the required hardware just it was blocked for specific countries

Happy FM transmitting