Q: How do I access the organiser's music from my account?

He is the System Administrator and has probably placed you in a Parental Control account which restricts what you can do and access. He likely does not want you using his credit card. Nothing we can do here about that, and in fact, we are not even Apple employees.

This link to accepted payment methods in the iTunes Store should help you. Just get a prepaid iTunes store gift card, and apply that to the iTunes purchases.

Also, the Apple Support Communities have several iTunes related communities for future information.