You go on the floor with the old dude and you can see the way up to Jasmine, the window you cant see, like with the light shining out of it, go through that. It's actually a door.
Well if it's the same in Heart Gold and Soul Silver you go up the lighthouse til you reach the top the the Pokemon will up there and the Jasmine girl will tell you to go to Cianwood City then you go...
First go to the top, find out the Pokemon is sick, and take the elevator down. after that, go to Cianwood City and go to the pharmacy. The man will give you medicine. Take it to her and she'll go to...