Duskull is a Generation III Pokémon and was introduced in Ruby, Sapphire, and Emerald. It wasn't available in the Generation II games like SoulSilver.
You can, however, find Duskull's pre-evolution, Sableye, in SoulSilver. You can find Sableye in Route 120, Victory Road, and the Safari Zone.
If you're looking to catch Duskull, you'll need to play a Generation III game!