How do you breed pokemon in Pokemon Soul Silver?

You can't. You can only breed in Gameboy or DS games. The Daycare in Agate Village is just a Daycare and not a Breeding Center.

- New Edit By Kaymon XD, Dont Steal!

Alright, You can... Only what you need is...

Emerald/Sapphire/Ruby/Firered/Leafgreen which is at the Daycare Center!

Then you need the GBA/GC Controller...

Then have a GBA SP/GBA

Step one : Plugging it In... Just plug in your GBA To the GBA/GC Cord, Put in (GBA Pokemon 3rd Gen or Remake of 1st Gen Here) Then go to the next step!

Step Two! : Turn it on, Turn on your Gamecube/Wii and play Pokemon XD and Turn on your GBA with that!

Step Three : I dont know that much about trading DX!!