Not a dumb question, at all. I asked the same question myself. This was Coop's answer:
The fuel system [in the SR22] is custom, so the X-Plane fuel tank switch commands wouldn't switch them. If you go into the command browser with the SR22 open, look for under afm -> sr22 -> cmd -> switches "Fuel Selector Left" and "Fuel Selector Right". These will move the handle position left and right. the order is OFF <-> LEFT <-> RIGHT <-> OFF
When I did it, I assigned the Right Tank to the keyboard letter R, and the left tank to letter L. I didn't try to make it a joystick button command, so if you want to use a joystick button, your solution may be a bit different.