MBOX 2 Mini and GarageBand set-up Please Help! MAC NEWBIES!

I am trying to set up my daughters MBOX 2 Mini with her garageband. She just wants to sing/rap over instrumentals. I can import a instrumental onto a track, but when I ad a track for the vocals, we can only hear the vocal as she sings into the mic ( and only if headphones are plugged into the MBOX ), but we cannot hear the instrumental unless we turn on the monitors that are connected the Mac. How do you connect everything so you can hear the vocal being recorded while the instrumental plays through one listening device, i.e. monitor headphones. I seems like I just may have some input/output issues.

Thanks for anyone that can help!

JDE