Instructions
Trade with a player using the "Diamond," "Platinum" or "HeartGold" edition of Pokemon to acquire a female Mantine. You can not catch Mantine in the wild on "SoulSilver."
Acquire a Wave Incense. You can find one in "SoulSilver" in Johto on the ground on Route 47.
Give the Wave Incense to your Mantine to hold.
Catch or take from your computer any male Pokemon compatible for breeding with Mantine. Mantine falls in the "Water 1" breeding group. Alternatively, you can use a Ditto.
Drop the Mantine and its partner off at the day care on Route 34. Continue on with your game and check back after a while to see if the Pokemon have laid an egg. Once they do, pick it up and carry it with you until it hatches into a Mantyke.