You need to get a credit card in the game first. Now this one, you'll need to keep that cord you used to hook up the GB/GBC/GBA but change that port to the N64. What you must do is do the Luigi cheat in Mario 64, and make Luigi jump on Mario's head 64 times in a row. Than Mario will yell "Ow! A-stop eet! Here's my a-credit card!" and in the other game you must go to NPC #823 and hook up the N64 to the system the game is on while talking to that NPC. But remember- you must be EXACTLY 500 steps from the NPC before talking to him, you can't move at all until Mario gives Luigi his credit card in Mario 64, and after Luigi gets the credit card you have two seconds to run up to the NPC, but you can't equip the 500-step boots or teleport.
Do the Mario! Swing your arms from side to side, come on, it''s time to go do the Mario! Take one step, and then again, let''s do the Mario all together now! You got it! It''s the Mario! Do the Mario! Swing your arms from side to side, come on, it''s time to go do the Mario! Take one step, and then again! Let''s do the Mario altogether now! Come on now! Just like that!