FM radio tuners are easily enabled when inserting modules. Simply
$ modprobe bttv radio=1
...or add a similar option to your /etc/modules.conf
and you should be set the next time you restart or load the module. You will
also need an application to access the radio. I suggest the
console based radio, available with the
Xawtv suite, or
fmtools.
For graphical programs, try gnomeradio
or gradio or xmms
with the FMRadio plug-in.
Also, you can't listen to radio at the same time as watching TV with your Bttv
card (unless you have more than one card, that is).