Took me some frustration to figure this out: you need a small flathead screwdriver (just a notch above watch sized, probably the smallest normal screwdriver you own), and a pair of needlenose pliers.

The pliers are just to hold the valve stem and prevent it from moving around. Position the valve so you can see into it, and pop the screwdriver into one of the notches on either side of the protruding pin. Then just unscrew it as you would a screw, while holding the stem with the pliers. The whole business should pop out pretty soon, and you can screw it back in the same way.

This thread is ancient, but I found it trying to figure this out, so maybe this will help someone in the future...