you have to finish Fiery Field (Entei) to unlock Lightning Field (Raikou)
to unlock Fiery Field, you must obtain HM Surf from Solar Cave (located in B20F, you'll find a room with water surrounding the item, you must bring a pokemon that leviatates from the ground ex. magnemite, wingull, latios, latias, etc. OR you must have the IQ All Terrain Hiker), then spinda will visit you and tell you about a mirage pokemon,