Oops, I see now. I just didn't realize the location of the door behind the big pillars. I thought Corey would be in the room upstairs.
Sorry for asking this (a bit silly) question, hehe..
@all: thanks
@sandiclaws: maybe (similar to me) you also didn't notice the real door (look at the right corner, behind the pillars). Or you maybe facing a different problem (in that case, I think it is better if you can share your save file here, so that others can help you).
@MEga DAVE: I got the egg by exchanging a Growlithe egg with the police officer in Jasper Ward, hehe.