you could go to a apple store, there are folks who can help, you MUST tell them that you are working on a iPod married to the bike that has that connection, newer stuff wont even plug in - they may try and get you something newer and that will be way way way harder to make work. you really want the iPod that came with it, they are reliable and durable and cheap.
the big problem is that "iPods" are not made any more, but they are supported.
it has to be connected to a computer with iTunes - mac or windows, both will work. that computer has to be connected to the internet.
once the iPod is synced to the computer, then you can put whatever music you want on the iPod, to make it ace most reliable purchase music from iTunes. there ARE ways to get music from CD's etc onto the iPod but you need to be computer savvy to do it.
its also important to understand that "computer wise" its old, but reliable tech, so kids probably have no idea how to make it work.