Instructions
Enter the large building on the east side of town. You'll find a bald man inside. Speak with him, and he'll task you with delivering a package to the leader of Team Magma.
Go to the Oceanic Museum, which is located on the eastern shore. Team Magma is holed up inside.
Speak with the grunt and he'll fight you, summoning a level 15 Numel. Counter it with a water or ground Pokemon. He only summons the one Pokemon, so go all out against it.
Heal your Pokemon and then speak with the second grunt. He summons a Zubat followed by a Numel. Defeat the Zubat with an ice or psychic Pokemon, then kill the Numel using the same water or ground Pokemon you used in the previous battle.
Speak with the last of the Team Magma members. This is Captain Stern. After a brief conversation, he takes the package and then leaves. You can now use the town's north exit to Route 110.