You shouldn't put websites in your posts (yes that is it).
I just found this:
First, go to Viridian to talk to the old man who shows you how to catch a pokemon. Then go the Cerulean City to get through the unknown dungeon. use repel spray to get through, as running into a wild pokemon will mess up the code. When you get to the area Mewtwo is usually located. you will find Yoshi. Unluckily, the only ball that can catch him is the master ball. Yoshi wont be listed in the Pokedex.
I will try this and post the results.
Romney '08!