in platnuim I beat the elite four, went to pal park, went to other sites, and got the national pokedex. I have NO idea what to do. I really apperiate an answer soon, I really want migrate GROUNDON, KRYORGRE, AND RAYQUAZA.
You have to put Emerald in the bottom slot of a Nintendo DS/DS Lite (if you don't have a DS or DS lite then you can't migrate. 3DS, 2DS, and DSi systems lack the ability to play GBA games), and start Platinum. At the startup menu there should be an option to "Migrate from Emerald" or something similar. Select that and you'll be able to migrate.