Quote from ToastyCactus
This is not true, you don't need to wait 2 days for anything to spawn, and slimes will now spawn up to Y:40.
First, make sure you're actually in a slime chunk, then light up any caves around you. Stand at least 24 blocks away from the pad, and if spawn rates are still slow, try going to the surface and killing off some passive mobs. You will get them eventually.
Lighting up the caves isn't really necessary and you'd have to do way too much work as they can spawn pretty far away from you. Passive mobs don't affect hostile mob spawns on the xbox. Passive(animal) and hostile mobs have their own separate limits.
As long as you find a slime chunk you'll be good to go. Don't dig out a massive area until you've found a slime otherwise you'll probably be wasting your time. You can use your world seed and a "program" to find slime chunks although it could be considered "cheating". Here is a slime chunk finder, just be aware that because of the 1000 world size your world only goes to -500 to 500 (on x and z).