Dark Pulse (move) is not included at Pokémon Emerald (Generation III), as it was introduced in Generation IV (TM79); TM97 in Generation VI.
What we found in Generation III is the Water Pulse (move) (TM03) located at Sootopolis Gym, it has a 20% chance to confuse the target.
Pokemon Emerald Version TM List: (Dark)
TM12 - Taunt - Location: Trick House
TM41 - Torment - Location: Slateport City
TM46 - Thief - Location: Slateport City
TM49 - Snātch - Location: S.S. Tidal
If you are having troubles at Pokemon Emerald here's the complete TM/HM List (Emerald) shared by Pokemon.neoseeker.com.