Before you start your pinball game, you will see a roulette spinning. When you press A, it will stop on a specific field. You can only catch certain types of Pokemon on certain fields. To get the one you want, press Start while its spinning. If it is the field you want, hold A. If it did not stop on the one you wanted, press Start and pause it again. It should stop on the same field you saw when you paused it. If it did not stop at the same field, restart your game and try again.