Pixelmod for PE

Before spawning in a world you will get an option to select which Pokémon you want to start out with.
To spawn a Pokémon press the P button in the bottom right corner of the screen. A list of Poké Balls will appear on the right side of the screen. Long-tap on either of the Poké Balls to spawn the Pokémon. (In this case, you will just have one Poké Ball.)
To catch Pokémon you will need some Poké Balls. There are three different types of Poké Balls and each of them have different chances (%) of catching Pokémon.

In this case I got an Ultra Ball. Throw it (long-tap) at a Pokémon (like a snowball) to catch it.