You have to talk to the scientist guy at the ship place in slateport city
Chat with our AI personalities
You first meet Capt. Stern upstairs in the museum, where team Aqua/Magma (I forget which) are originally blocking the doorway.
To get to Mauville, you need to do a couple things. 1) Attempt to drop off the parts at the shipyard by talking to the man standing in front of the blueprints. He will say that you should go find Capt. Stern. 2) Now, go to the museum. Notice there are no longer any Aqua members blocking the entrance. Walk inside. But, before you go upstairs, talk to the Aqua Grunt closest to the door and he will give you TM49 (Theif). NOW head upstairs. 3) A couple of Aqua's will attempt to steal your package after talking to Stern, but once you beat them, he will take the parts, and the road to Mauville will be open. Just head North out of the city until you reach the Trick House. Once there, turn to your right. This is the road to Mauville.
Capt. Stern is on the top floor of the museum. You can't get in though, because there is a line of Grunts blocking the entrance.So first, you need to go to his Shipyard (still in Slateport) and talk to the guy north of the door. His name is Dock, and he'll tell you to deliver the Goods to Stern.When you go back to the museum, the Grunts are gone.You need $50 to get into the museum.
He's in the Slateport museum but you might have to go talk to a guy in the big, brown building (go up from the entrance and he's in front of some blueprints).
when you are goin to deliver the Devon goods go to the oceanic museum if there are still team aqua lining up outside go to stern's shipyard and talk to dock near a blueprint of a ship then go back to the museum pay P50 first for the entrance fee and then talk to a team aqua and will give you a tm46 thief and he will run away go upstairs if see a man near the replica of s.s.anne talk to him because his capt. stern!! and two team aqua grunts will battle you! and give the Devon goods