You have to go up the ladder, and at the floor with a dead end, Look for a window that is bigger than the others, it is a door.
The lighthouse is in Olivine City, where the gym leader Jasmine is.
find a way to them talk to jasmine get out of the lighthouse go back to cianwood city and go to the pharmacy talk to the guy and he will give you the medicine
You need to have seen Jasmine in the Olivine Lighthouse in Olivine City first. You'll find her at the top floor of the lighthouse.
find jasmine at the top .to get to the top go up ladders .
you have to go to cianwood city and go to the top of the lighthouse and find jasmine she will give you the secretpotion.
you keepon going up the stairs and ladders in thelighthouse
she will be in the olivine lighthouse on the top floor. search walkthrough's to get there easily
The lighthouse is in Olivine City, where the gym leader Jasmine is.
if you mean the olivine lighthouse you have to beat all the trainers to get to the top, talk to jasmine, go to cianwood to get the secret potion then surf back then give it to jasmine then the Pokemon will be fine then you can defeat chuck or do jasmine your choice i did chuck first cause he is really easy!!!
After talking to Jasmine in the Lighthouse, you have to go to Cianwood City and get a Potion from the Pharmacy there. Take it back to Jasmine and she'll go back to her Gym.
Get the secretpotion from a guy there and go back to Olivine in the lighthouse and cure the Ampharos in lighthouse and then battle Jasmine
Go get the SecretPotion from Cianwood and give it to Jasmine at the top of the lighthouse. You can then battle htr. or if you cant get to her where it looks like a window; there is a door.