With most pedals you need to run them along with another analog joystick.
My solution has always been to run the pedals along with a regular analog steering wheel. Of course, I recently removed my joystick port so I had to buy new USB pedals and no longer have the option to hook up my old analog wheel unless I reinstall the old soundcard.
But the bottom line is that you generally need to run the pedals with a regular joystick for it to be recognized. Some folks hacked together a "z-axis only" stick driver that allegedly would recognize rudder pedals all by themselves, but that driver never worked for me.