You CANNOT TRADE Pokémon between the GBA & DS versions.
YOU CAN, however, import Pokémon from the GBA versions into the DS versions through the Pal Park. Please note that you'll need to complete the game & have the National PokéDex in order to open up the Pal Park.
When you're ready to import your pokémon, select the "Import from ..." option. Then select the 6 pokémon you want to import to th Pal Park. Please note that the pokémon you want cannot have any HM moves. You'll have to go to the Move Deleter beforehand.
Once you have 6 pokémon selected, hit import & they'll be moved from the GBA pack into your DS's Pal Park.
IMPORTANT: THIS IS A ONE-WAY DEAL! DO NOT IMPORT POKEMON TO THE DS VERSIONS UNLESS YOU ARE CERTAIN THAT YOU WILL NO LONGER NEED THEM IN THE GBA VERSION!
Please note that you can only import pokémon one per GBA pack per 24 hours. If you change your DS or Pokémon's internal clock (in the GBA version), you'll have to wait an additional 24 hours.
Once you have the Pal Park stocked, go to the Pal Park & play the "Catch 'em" minigame! Enjoy!