You have to be playing games that support that extra button of course.
assuming you are, you should have a working B and C buttons, this is true for the 6 button pads too (which I use) Some games which should support the extra button in WHDload don't seem to support this very well however, and one of the faster Master System emulators also does not support it, despite saying it does. So perhaps something does need to be done to the pads to make them work consistently. I recall one post somewhere, where someone suggested it was because the rubber contact pad has different resistance properties to a microswitch, but in this case i would have thought that no buttons or directions on a pad would work reliably.