You Have to type the stadium codes from the stadium 
select screen:

Cornfield Stadium 
From the Stadium Select screen, hit this button combo: 
UP, DOWN, RIGHT, UP, DOWN, LEFT, UP. You should see 
the stadium from Field of Dreams. 

Mystery Stadium 
From the Stadium Select Screen, hit this button combo: 
RIGHT, LEFT, UP, LEFT, DOWN, RIGHT, LEFT 

Special keys 
Monster Home Run: hold down 1,2, and 7 and you will 
hit a homerun at least 570 feet, even if you BUNT!