you use your computer, not the n64
download it (called project 64) then you simply google for a ROM of the N64 game you want (e.g google 'mario kart 64 rom) and download it and save in the appropriate place (sure it tells you how somewhere) and then you can go to options in project 64 to configure any controllers you plug into the computer (we use 360 controllers and PS3 controllers, any standard usb controller should work) and you just launch whatever game you want, that simple
occasional crashes but mostly fine and its free.
Nintendo 64 Emulator - Play N64 games on your Windows PC