First dont battle the kid with the slowpoke on route 24 or 25.Also you must have a pokemon that knows teleport.Then you will see a bug catcher on the side of route 24,go right in front of him,then quickly press the start butten then use teleport then go battle the kid with the slowpoke.After your done fighting him use teleport.Then go up the nugget bridge and the start menu should apear, exit out of the start menu and mew will apear!