This will be my last silly and embarassing question....but, am I supposed to plug in my electric guitar into an amp, and then into my Fasttrack USB?

Not a user myself, but I think it's designed as an instrument interface pitched at guitarists in particular. You plug your guitar into it, send the signal into something like logic and use a guitar emulator plug-in to make sounds. It also has a little standalone application with it so you can muck around without using any sequencer/DAW software (or by rewiring it into your sequencer if you like the sounds). If you want to record your real amp sounds you will have to use a microphone and go through the mic inputs.