once you beat the Elte Four you need to surf to the bottom of Sandgem town (where Dawn lives) there you wwill find the pal park.
1 turn offther game put the Leafgreen game on thre bottom of the DS and then turn on the game
2choose pokemon pearl and there should be a new option it should say Migrate from Leafgreen or something similar
3 once you choose that option you can migrate 6 pokemon nomore no less to pokemon pearl
4 then once you start the game you can capture the pokemon you migrated in the pal park
NOTE:you can only migrate 6 poemon a day from any other game
once traded those pokemonn cannot be traded back to that game
also this the only way you cant trade between pokemon d/p and LG/FR/R/S you can only migrate