Instructions
Turn on your Wii and Nintendo DS with the appropriate games inserted. Press any button to go to the Game Mode menu on the Wii.
Click on DS Battle Mode in the Wii menu. Select the game and and click on "Start Entry." Now on your Pokemon menu on the DS, it should have an option to "Connect to Wii." Click it.
Go back to your Wii menu and press "Start" when it prompts you. You now have the DS and Wii connected.