the stem should wiggle or lift straight out. Then turn the bike upside down and the other piece should fall out too. I can't find a picture right now to show you, but it's in there. If it doesn't fall right out, take the bolt out of the stem and thread it part way into that other piece, then pull it out that way. Liquid Wrench or PB Blaster may come in very handy here.

ETA: Found the picture! See the wedge-shaped piece on the bottom? You're looking for something like that: