Plese Help Me With My First Set Up!, General questions and help with hardware Options

[depakote]

I have recently purchased a PCR-30 Edirol Midi Keyboard Controller, a US-122 USB Audio/Midi Interface, and a pair of self powered speakers by M-Audio to use with my Powerbook. I am using Garage Band to get started. The keyboard can be powered via USB or though the AC adapter and the interface is powered strictly by USB. When I hook the keyboard up via USB I can control the virtual instruments on Garage Band but when I run the keyboard through the interface I can't. The Midi In light comes on telling me the interface is getting the signal but it doesn't make it to the computer for some reason. The interface features direct monitoring but I suppose that that doesn't matter when you are using virtual instruments? What is the function of midi in on a keyboard controller? I have donwnloaded all the latest drivers and also set the input and output to the interface. I suppose I could just run the keyboard into the computer via USB and run the audio via the line output of my computer but assume that this would mess up direct monitoring for mics and guitars? I also want to use the speakers to play music I have stored on the computer from itunes. Is there a way to send the music through the keyboard to the interface to my speakers to hear it or do I just have to run the audio through the line out of the computer? Any help greatly appreciated. Thanks in advance!