The Wifi infrastructure has no specific support for FM audio. You will need a special device for that. Search Amazon for "home fm transmitter" for ideas. Such a device has to plug into an audio source, which could be the audio output on your MacBook or an AirPort Express base station.
If the stations that you want to receive are also available over the internet, you could use Airfoil to stream the signal from your computer to your AirPort Express.
Just to state it a little differently, the wireless abilities of you MacBook have nothing to do with FM. Therefore the MacBook has no ability to transmit or receive an FM signal.
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Using airport as fm transmitter?
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