FM radio in the Defy comes bundled in the music player. Firstly, connect the wired headset, because it will act both for listening to the FM radio, and as as the FM antenna itself. Ok, either go to its icon, (in the 6th home screen), or go to 'Start (the circle in all the home screens) > Music'. Then, if the last music you played comes up, press 'Menu > Music Home'.
Before you click on 'Radio', on the home screen, if you are worried about data usage, be warned that it will occur if you want lyrics to be shown as music is being played. If you want that feature, but don't want to use your carrier's data plan, you can turn your phone's Wi-Fi on, and connect to either your home network or a plublic access point. If you need further informations on how to do it, please, let me know. To turn this feature off, go to 'Menu > Settings > Display lyrics' and make sure that 'Show lyrics in player' is unchecked. Press the back button, then 'Data Usage', where you'll find checkboxes for 'Download song data' and 'WiFi only purchasing'. Set them accordingly. Press the back button twice, then 'Radio'. You are now listening to the FM tuner. After finishing, press the back button until you reach any of the home screens.
You'll notice that the embedded FM player is able to play sound on the wired headset only. It this is if fine for you, we're done here. Neverteless, this standalone application
(many thanks to yipyipyip)
doesn't even require that the wired headset be plugged in, at all. If you are not subscribed to xda-devs, do it. Download link in the first post. Download it and copy it to anywhere on your phone (e.g. sdcard). Then, go to 'Menu > Settings Applications', and make sure that 'Unknown sources' is checked. Press the back button twice to finish. Now, go to 'Start > File Manager > Memory card', and navigate to the apk file you have downloaded. Click over it, and you will be prompted to install it. Then, go to 'Start > FM radio' and have fun!