How do I connect and play my electric guitar through my iMac (OS X Yosemite)

Sorry if this post is long, but I am really frustrated. I have reead multiple posts and tried multiple things suggested all to no avail. To save time for responders I am listing everything I have tried.

I have a new iMac with Yosemite (10.10.3). I want to be able to connect my electric guitar to my iMac and play along with iTunes and Online Guitar lessons. I have tried two methods of connecting my guitar to my iMac: A Hosa TrackLink Guitar to USB Interface cable and a M-Audio Fast Track Guitar/Mic recording Interface box. The M-Audio used to work fine with my Macbook running Snow Leopard and the older Garage Band. When in System Preferences I set the Sound input to either device, I can see signs of Input Level when I strum the guitar, however I hear absolutely no sound from my speakers. I have tried both the Internal speakers and the headphone output to my external speakers (which, by the way, work fine if I platy a song on iTunes) - no guitar sound in either case.

I have read all the posts I can find and none of them tell me anything that solves the issue. I have seen references to turning Monitor On in Garage band. I looked but saw no such option. I may be a bit ignorant with Garage Band because I haven't used it since the new version appears to be focused on recording and, as I said, all I want to be able to do is plug in and play along with iTunes and online guitar lessons.

What am I doing wrong here? Do I need some sort of App link Amp or will Garage band actually do what I need without recording? Can someone help a dummy out?

Thanks!