Instructions
You must own a Wonder Card from an official Nintendo event to enter the Yellow Forest. This can be done by visiting an official event location and selecting "Mystery Gift" from the main menu. Select "Receive a Gift" and your DS Wi-Fi will detect the gift for you.
From the Mystery Gift menu you can check your Card Album, which displays all the Wonder Cards you own. Check that your album contains the card "The Yellow Forest."
Select "Connect to Pokewalker" from the game's main menu. Select "Save, then go for a stroll."
Choose a Pokemon from your PC storage to transfer to your Pokewalker. You may now choose a map to explore. With the Yellow Forest Wonder Card, the option to enter the Yellow Forest will appear.
Press the middle button on your Pokewalker when you're prompted to, and point the Pokewalker's Infrared receiver at the top of the DS. Wait five seconds. Your Pokemon is now transferred and you may enjoy the Yellow Forest.