Griffin Technology

I've plugged my guitar up to my computer using the Cable, but I don't see anything in my input level.

First, make sure your computer has an audio input jack, not just a headphone jack. The headphone jack is an audio output and won't bring audio into your computer! If you're sure you do have an audio input jack, determine if that input is line or mic level. The GarageBand Cable matches impedance, but it's not going to boost a mic-level signal. You may need an additional preamp (such as the iMic) or a powered mixer to achieve enough volume.