You can only migrate from the GBA Games (Leaf Green, Fire Red, Sapphire, Ruby, Emerald). If you want pokemon from D/P/Pt then you trade them over from DS -> DS. Here's how to migrate pokemon:
~insert GBA game into GBA cartridge in your DS
~get into your MENU (where it has your saved data, mystery gift, ect.)
~go down to "Migrate Pokemon"
~it will then take you to your PC boxes in your GBA game
~chose 6 pokemon that don't know H.M.'s or they can't be migrated
~go into your Soul Silver saved data
~fly to Fuchsia City
~go into the Pal Park
Well after that you enter the Pal Park and you will catch the pokemon you've migrated!!