Slimes only spawn in certain chunks of the map. Chunks are 16x16x128. Just look for signs of slimes like their noise or possibly even slime balls from deadslimes. If you find any of that, just clear a big 16x16x3 room.
According to the Minecraft Wiki, they spawn in the lowest 40 layers of the map, and only in certain areas. Search for "Minecraft slime farm" on Youtube, there are several good videos that explain everything. And i see you are a pretty big fan of slimes.
Actually, at this point in the Minecraft versions, Slimes only spawn in the bottom 16 layers of the map, not the bottom 40. That was added in a 1.9 pre-release so we won't get it until 1.0 on the Xbox version.
For best results, make a 3 high room (or bigger). If you can extend it several chunks you have an even higher chance of slimes.