Instructions
Travel to the Pokemon Center on Route 10, to the east of the Rock Tunnel.
Walk north from the Pokemon Center to the river.
Press "A" next to the river. Select "Yes" to begin surfing.
Travel south down the river until you reach a large building. This is the power plant.
Surf into the ground near the power plant to automatically stop surfing. Walk into the doors on the front of the building to enter the power plant.