Instructions
Catch a Slowbro or evolve a Slowpoke by training one to level 37.
Find a King's Rock and let your Slowbro hold the item. You can find a King's Rock on wild Poliwhirls, which are found on Routes 225, 227 and 228 when fishing with a Good Rod or better.
Using the inter-game trading system, trade a Slowbro carrying a King's Rock to another "Pokemon Pearl" game. After the trade goes through, Slowbro will evolve into Slowking.