Instructions
Head for the large building on the right edge of Slateport City and speak to the bald man inside. He will tell you to take the Devon Goods to Captain Stern, who is in the Oceanic Museum. The Team Magma members who were initially blocking the path into the museum will be gone after you have this conversation.
Go to the museum and pay the admission fee to enter.
Speak to a member of Team Magma to get TM46 Thief and then walk upstairs to speak to Captain Stern. After you talk to him, a fight against the two grunts beside him will begin.
Defeat the first grunt's Pokemon, Numel, using a Fighting, Water or Ground type Pokemon. Use a Psychic or Ice type Pokemon to defeat the second grunt's Pokemon, Zubat. Once you win the battle you will be able to travel north along Route 110 to visit the next area.