Instructions
Acquire the "Seismic Toss" from the move tutor in the "Pewter Museum" in the "Pokemon FireRed" video game. This move costs two small mushrooms or one large mushroom.
Turn off your Nintendo DS and insert the "Pokemon Firered Version" cartridge into the GameBoy Advance slot on the bottom of the Nintendo DS.
Insert "Pokemon Diamond" into the regular DS cartridge slot on the Nintendo DS.
Turn on the Nintendo DS.
Select "Play Nintendo DS."
Select the "Migrate From FireRed Version" option on the main menu of "Pokemon Diamond."
Select the "Yes" option twice.
Select six Pokemon from the available list that you wish to migrate from the FireRed version to the Diamond version. Make sure that you include Blissey in your selection. You must select exactly six Pokemon for the migrate to work properly.
Confirm the migration by selecting "Yes" twice and then wait for the game to reboot.
Go to the "Pal Park" in the "Pokemon Diamond" game once it reboots by following "Route 221." Once you arrive, walk into the building and speak with the man inside. He will give you six Pokemon balls. Walk outside and use the balls to capture the six Pokemon that you have migrated. Blissey will now be available for use with the "Seismic Toss" move.