I use a Line 6 Tone Port UX1, its a box the has guitar and mic inputs with a usb out to the computer. Musicians Friend is a great place to find boxes like this. Here is the link to my model: http://www.musiciansfriend.com/prod...X1-USB-RecordingModeling-Interface?sku=249700
http://www.amazon.com/Griffin-Techn...1?ie=UTF8&s=electronics&qid=1197320750&sr=8-1
I have this and it works like a charm. I've tried the cheap 1/4 -> 1/8 converter but it was very problematic unlike this cable. It is a great first way to get your guitar hooked up to your mac.
Try the quarter inch to 3.5mm adapter before you splash out as it can be had for under 10 of your puny $.
That looks nice and simple... Is the sound quality good?? Are you sure that will plug into the line in of my MacBook?
Absolutely sure, as I use it with my MacBook. Once you plug it in you'll have to change settings in GarageBand and SystemPreferences to use the Line In instead of the built-in microphone, and you'll be all set!