I entered the Pokemon code for Groudon in right and it accepted it and I went to the right place to find but he wasn't there at all. I tried to find him three times already.
Well, from the sound of things.... There's probably something wrong with your game.
So double or triple check your code ( Again ) and lok around thouroughly, if you do so and he still isn't there; your game is glitched.
He's toward the indent in the eastern wall. (At least that's where I found mine.) Try running around a bit there. If he doesn't show up, you have a glitched game, sorry to say.